Hey there @SuperbPetuch! Welcome to the community! For many use cases as long as the final product is inside of Unreal, modifying any of the assets outside of engine is fine but you have to use them inside of Unreal and anything distributed must have been through the engine. Double check your specific use case against the EULA as this is just an interpretation, I am not a lawyer, and different use cases can be affected by different portions of the EULA.