If you're asking about entity access and specialization of entities, it's quit easy, but a need to know. Setting up the entity access can be read in the Access Rules post and the how to Create A Secure App. But how on specialized entities? Each entity is unique for Mendix with or without generalization. The rules on Vehicle are different from Car and Truck, the rules from Car are overwriting the rules from Vehicle. So you can set that an user has write rights on the 'Numberplate' if the entity is a Car but have read rights when it's a Truck and no access at all when it's a Vehicle.
I hope this will give you some insight on the entity acccess within Mendix in relation to specialization.