Why do some contacts have a Personnel tab, and other have a Personal tab instead?

There are two types of contacts that you can enter on any of the contact windows; Companies and Individuals. To the right of the contact name field you will find a checkbox labelled "Individual".

When this box is checked, the contact is an individual person and the Personnel tab is replaced with a Personal tab which contains personal information for the individual.

When the Individual box is unchecked, the contact is a company and you will see the Personnel tab where you can enter all of the individuals that work for the company.