I've been researching and found that
The recipients are anything you can send mail (eg mailbox, group, resource, contact etc ...)
The mailbox is the conbinacion of a user directory and a reserved space in a database where information as exchage (email calendar, history, attachments etc ...) are saved.
My question is when I modify add-recipientPermisions to send as;
¿I am modifying the permissions to the address of mail which is stored in active directory?
And when I use add-mailboxPermisions
¿I am modifying the permissions of the area reserved in the database that stores emails?