Ok - you have a physical domiciled residence that you use with the state for voter and title registration - example --
231 Lake St,
Anytown TN 3XXXX-XXXX

But your town - Anytown does not do delivery so you have
PO BOX 123
Anytown TN 3ZZZZ-ZZZZ

However the Station Address is
987 Main ST
Anytown TN 3QQQQ-QQQQ

Here it would be
987 Main ST UNIT 123
Anytown TN 3QQQQ-QQQQ
or
987 Main ST STE 123
Anytown TN 3QQQQ-QQQQ
or
987 Main ST #123
Anytown TN 3QQQQ-QQQQ
or
987 Main ST PMB 123
Anytown TN 3QQQQ-QQQQ
or
987 Main ST APT 123
Anytown TN 3QQQQ-QQQQ

verify at http://www.usps.com/zip4