Many windows in ActivityHD have one or more reference fields. Reference fields are easy to spot because of the button with the right-pointing arrow 
at the end of the field. Clicking the button opens a menu with additional commands. In particular, one of those commands is the Find command. You can also access the Find command by pressing F3.
When you select the Find command or press F3 on a reference field, the Find dialog box for the referenced entity opens.
The Find dialog box is like a condensed HD view, including the Filters and Lookup fields and the Action mini-toolbar. Likewise, the columns in the Find dialog box are sortable just as in the HD view.
Tip
The Find dialog box supports incremental search. With focus in the list box, start typing the first few characters of the identifier for the record you are seeking. ActivityHD moves to the first record that matches your input.
The main purpose of the Find dialog box is to help you look up and select the value you want to enter in the underlying field. To enter a value in the field, either double-click it or highlight it and click OK.
If you don't see the value you need, you can click 
to add it.
If you're unsure about the value you need, you can highlight a value and click 
to open the record in read mode and see its setup and which other records reference it.
You can also highlight a record and click 
to edit it or 
to delete it.