Normally, when you select a choice from a Lookup List, your choice overwrites whatever was already in the data field. The semicolon allows you to add your selection to the already existing data without removing the original data.
Pull up the Lookup List you want to modify, choose the field entry to modify, and click Edit. After the data in the Enter the F2 Value box, add a semicolon (;). Click OK to save.
That choice in your lookup will now go into the data field immediately after the data that is already there, separated by a comma and a space. Using this technique, you can have multiple selections in your data fields.
Tip: When using multiple selections, be aware that you can sometimes run out of space in the data field. Comma and spaces count toward field length along with the actual number of characters. Know your data field length and edit accordingly.
Also, using semicolons will not work if you are using the Force Valid Input option in the F2 Field Setup box. If you want to use semicolons, pull up the Lookup List you want to use them with and go to Setup; then uncheck the Force Valid Input box and hit OK.