diff --git a/src/newgrf.cpp b/src/newgrf.cpp
--- a/src/newgrf.cpp
+++ b/src/newgrf.cpp
@@ -5508,8 +5508,8 @@ static void NewSpriteGroup(ByteReader *b
 static CargoID TranslateCargo(uint8_t feature, uint8_t ctype)
 {
 	/* Special cargo types for purchase list and stations */
-	if ((feature == GSF_STATIONS || feature == GSF_ROADSTOPS) && ctype == 0xFE) return CT_DEFAULT_NA;
-	if (ctype == 0xFF) return CT_PURCHASE;
+	if ((feature == GSF_STATIONS || feature == GSF_ROADSTOPS) && ctype == 0xFE) return SpriteGroupCargo::SG_DEFAULT_NA;
+	if (ctype == 0xFF) return SpriteGroupCargo::SG_PURCHASE;
 
 	if (_cur.grffile->cargo_list.empty()) {
 		/* No cargo table, so use bitnum values */
@@ -5637,9 +5637,9 @@ static void VehicleMapSpriteGroup(ByteRe
 		EngineID engine = engines[i];
 
 		if (wagover) {
-			SetWagonOverrideSprites(engine, CT_DEFAULT, _cur.spritegroups[groupid], last_engines, last_engines_count);
+			SetWagonOverrideSprites(engine, SpriteGroupCargo::SG_DEFAULT, _cur.spritegroups[groupid], last_engines, last_engines_count);
 		} else {
-			SetCustomEngineSprites(engine, CT_DEFAULT, _cur.spritegroups[groupid]);
+			SetCustomEngineSprites(engine, SpriteGroupCargo::SG_DEFAULT, _cur.spritegroups[groupid]);
 			SetEngineGRF(engine, _cur.grffile);
 		}
 	}
@@ -5722,7 +5722,7 @@ static void StationMapSpriteGroup(ByteRe
 			continue;
 		}
 
-		statspec->grf_prop.spritegroup[CT_DEFAULT] = _cur.spritegroups[groupid];
+		statspec->grf_prop.spritegroup[SpriteGroupCargo::SG_DEFAULT] = _cur.spritegroups[groupid];
 		statspec->grf_prop.grffile = _cur.grffile;
 		statspec->grf_prop.local_id = station;
 		StationClass::Assign(statspec);
@@ -6092,7 +6092,7 @@ static void RoadStopMapSpriteGroup(ByteR
 			continue;
 		}
 
-		roadstopspec->grf_prop.spritegroup[CT_DEFAULT] = _cur.spritegroups[groupid];
+		roadstopspec->grf_prop.spritegroup[SpriteGroupCargo::SG_DEFAULT] = _cur.spritegroups[groupid];
 		roadstopspec->grf_prop.grffile = _cur.grffile;
 		roadstopspec->grf_prop.local_id = roadstop;
 		RoadStopClass::Assign(roadstopspec);
diff --git a/src/newgrf_cargo.h b/src/newgrf_cargo.h
--- a/src/newgrf_cargo.h
+++ b/src/newgrf_cargo.h
@@ -14,9 +14,16 @@
 #include "cargo_type.h"
 #include "gfx_type.h"
 
-static const CargoID CT_DEFAULT      = NUM_CARGO + 0;
-static const CargoID CT_PURCHASE     = NUM_CARGO + 1;
-static const CargoID CT_DEFAULT_NA   = NUM_CARGO + 2;
+/**
+ * Sprite Group Cargo types.
+ * These special cargo types are used when resolving sprite groups when non-cargo-specific sprites or callbacks are needed,
+ * e.g. in purchase lists, or if no specific cargo type sprite group is supplied.
+ */
+namespace SpriteGroupCargo {
+	static constexpr CargoID SG_DEFAULT    = NUM_CARGO;     ///< Default type used when no more-specific cargo matches.
+	static constexpr CargoID SG_PURCHASE   = NUM_CARGO + 1; ///< Used in purchase lists before an item exists.
+	static constexpr CargoID SG_DEFAULT_NA = NUM_CARGO + 2; ///< Used only by stations and roads when no more-specific cargo matches.
+};
 
 /* Forward declarations of structs used */
 struct CargoSpec;
diff --git a/src/newgrf_engine.cpp b/src/newgrf_engine.cpp
--- a/src/newgrf_engine.cpp
+++ b/src/newgrf_engine.cpp
@@ -31,7 +31,7 @@ void SetWagonOverrideSprites(EngineID en
 {
 	Engine *e = Engine::Get(engine);
 
-	assert(cargo < NUM_CARGO + 2); // Include CT_DEFAULT and CT_PURCHASE pseudo cargoes.
+	assert(cargo < NUM_CARGO + 2); // Include SpriteGroupCargo::SG_DEFAULT and SpriteGroupCargo::SG_PURCHASE pseudo cargoes.
 
 	WagonOverride *wo = &e->overrides.emplace_back();
 	wo->group = group;
@@ -44,7 +44,7 @@ const SpriteGroup *GetWagonOverrideSprit
 	const Engine *e = Engine::Get(engine);
 
 	for (const WagonOverride &wo : e->overrides) {
-		if (wo.cargo != cargo && wo.cargo != CT_DEFAULT) continue;
+		if (wo.cargo != cargo && wo.cargo != SpriteGroupCargo::SG_DEFAULT) continue;
 		if (std::find(wo.engines.begin(), wo.engines.end(), overriding_engine) != wo.engines.end()) return wo.group;
 	}
 	return nullptr;
@@ -1044,7 +1044,7 @@ VehicleResolverObject::VehicleResolverOb
 	cached_relative_count(0)
 {
 	if (wagon_override == WO_SELF) {
-		this->root_spritegroup = GetWagonOverrideSpriteSet(engine_type, CT_DEFAULT, engine_type);
+		this->root_spritegroup = GetWagonOverrideSpriteSet(engine_type, SpriteGroupCargo::SG_DEFAULT, engine_type);
 	} else {
 		if (wagon_override != WO_NONE && v != nullptr && v->IsGroundVehicle()) {
 			assert(v->engine_type == engine_type); // overrides make little sense with fake scopes
@@ -1061,9 +1061,9 @@ VehicleResolverObject::VehicleResolverOb
 
 		if (this->root_spritegroup == nullptr) {
 			const Engine *e = Engine::Get(engine_type);
-			CargoID cargo = v != nullptr ? v->cargo_type : CT_PURCHASE;
+			CargoID cargo = v != nullptr ? v->cargo_type : SpriteGroupCargo::SG_PURCHASE;
 			assert(cargo < lengthof(e->grf_prop.spritegroup));
-			this->root_spritegroup = e->grf_prop.spritegroup[cargo] != nullptr ? e->grf_prop.spritegroup[cargo] : e->grf_prop.spritegroup[CT_DEFAULT];
+			this->root_spritegroup = e->grf_prop.spritegroup[cargo] != nullptr ? e->grf_prop.spritegroup[cargo] : e->grf_prop.spritegroup[SpriteGroupCargo::SG_DEFAULT];
 		}
 	}
 }
diff --git a/src/newgrf_roadstop.cpp b/src/newgrf_roadstop.cpp
--- a/src/newgrf_roadstop.cpp
+++ b/src/newgrf_roadstop.cpp
@@ -196,11 +196,11 @@ RoadStopResolverObject::RoadStopResolver
 
 	this->town_scope = nullptr;
 
-	CargoID ctype = CT_DEFAULT_NA;
+	CargoID ctype = SpriteGroupCargo::SG_DEFAULT_NA;
 
 	if (st == nullptr) {
 		/* No station, so we are in a purchase list */
-		ctype = CT_PURCHASE;
+		ctype = SpriteGroupCargo::SG_PURCHASE;
 	} else if (Station::IsExpected(st)) {
 		const Station *station = Station::From(st);
 		/* Pick the first cargo that we have waiting */
@@ -214,7 +214,7 @@ RoadStopResolverObject::RoadStopResolver
 	}
 
 	if (roadstopspec->grf_prop.spritegroup[ctype] == nullptr) {
-		ctype = CT_DEFAULT;
+		ctype = SpriteGroupCargo::SG_DEFAULT;
 	}
 
 	/* Remember the cargo type we've picked */
diff --git a/src/newgrf_station.cpp b/src/newgrf_station.cpp
--- a/src/newgrf_station.cpp
+++ b/src/newgrf_station.cpp
@@ -496,12 +496,12 @@ uint32_t Waypoint::GetNewGRFVariable(con
 
 	switch (this->station_scope.cargo_type) {
 		case INVALID_CARGO:
-		case CT_DEFAULT_NA:
-		case CT_PURCHASE:
+		case SpriteGroupCargo::SG_DEFAULT_NA:
+		case SpriteGroupCargo::SG_PURCHASE:
 			cargo = 0;
 			break;
 
-		case CT_DEFAULT:
+		case SpriteGroupCargo::SG_DEFAULT:
 			for (const GoodsEntry &ge : st->goods) {
 				cargo += ge.cargo.TotalCount();
 			}
@@ -557,11 +557,11 @@ StationResolverObject::StationResolverOb
 	/* Invalidate all cached vars */
 	_svc.valid = 0;
 
-	CargoID ctype = CT_DEFAULT_NA;
+	CargoID ctype = SpriteGroupCargo::SG_DEFAULT_NA;
 
 	if (this->station_scope.st == nullptr) {
 		/* No station, so we are in a purchase list */
-		ctype = CT_PURCHASE;
+		ctype = SpriteGroupCargo::SG_PURCHASE;
 	} else if (Station::IsExpected(this->station_scope.st)) {
 		const Station *st = Station::From(this->station_scope.st);
 		/* Pick the first cargo that we have waiting */
@@ -575,7 +575,7 @@ StationResolverObject::StationResolverOb
 	}
 
 	if (this->station_scope.statspec->grf_prop.spritegroup[ctype] == nullptr) {
-		ctype = CT_DEFAULT;
+		ctype = SpriteGroupCargo::SG_DEFAULT;
 	}
 
 	/* Remember the cargo type we've picked */