By default, a FSoftObjectPath property can be set to any asset. The AllowedClasses specifier limits the asset selection UI to show only those assets that derive from the enumerated types. The comma-separated list should contain type names without leading letters (e.g. Actor instead of AActor).
For instance, in the following example, the asset selection UI will only show assets that are of type UMaterialInterface.
UPROPERTY(Meta=(AllowedClasses="MaterialInterface"))
FSoftObjectPath MaterialPath;