Their email address is a unique field to this table and would likely be the table's primary key.
An email address is not a good choice for a primary key.
The serial number would be the primary key.
A better choice than an email, but still not the optimum to use for a primary key. The ideal primary key is: Internally generated by the database, unique, devoid of any additional "encoded meaning" and never ever changed. Email addresses fail this test primarily due to the last requirement, equipment serial numbers don't meet these requirements as they are not internally generated.
In FileMaker Pro, your primary keys should be either number fields with an auto-entered serial number or text fields that auto-enter the Get ( UUID ) (new feature in FileMaker 12) calculation.
I understand one-to-many and many-to-many relationships, and why all relationships need to be resolved to be one-to-many.
Right now this is a many-to-many relationship
If you understand these two key details, where exactly do you need help?
Start with these relationships:
Personnel::__pkPersonnelID = Personnel_Equipment::_fkPersonnelID
Equipment::__pkEquipmentID = Personnel_Equipment::_fkEquipmentID
You can place a portal to Personnel_Equipment on the Personnel layout to list and select Equipment records for each given Personnel record. Fields from Equipment can be included in the Portal to show additional info about each selected Equipment record and the _fkEquipmentID field can be set up with a value list for selecting Equipment records by their ID field.