Answer
At ArcPad 6, only a single applets path was offered and it was always displayed on the applets path input field.
In ArcPad 7.0 and 7.1 releases, multiple applets paths are offered. There is always a default applet path established during installation; on the PC it is:
C:\Program Files\ArcPad 7.0\Applets\ OR C:\Program Files\ArcPad 7.1\Applets\.
If an applets path is set, the Preferences.Properties, "AppletsPath", displays the applets path.
If an applets path is not set, the Preferences.Properties, "AppletsPath", gives an empty string. However, the default applet path is used by ArcPad.
Test this by opening the Script dialog box (Tools > Utilities > Run Script) and run the following script:
MsgBox [Preferences.Properties("AppletsPath")]