Access rights.

1. Every system user has got default access rights set by the Administrator.

2. The user can have read or read/write access.

3. The user can also provide access to his record to other users in Access tab of the record details.

4. If the user has access to the account (or the contact), but has no access to the sales (or tasks), linked to this account, then he will see only the account data, but not sales or tasks linked to this account.  

5. If the user has access to the task, he also sees all contacts, linked to this task. But he can open and edit them only if he has read or read/write access to these contacts

6. If the user is linked to the task or the sale, he automatically obtains read or read/write access rights to this sale (task). The Administrator can adjust this option in System settings

7 . If the user has no access to the invoice, he can't see it in the sale details