The operations required for configuration and usage of scanners for attendance is dependent on the type of scanner.
Keyboard Wedgestyle scanners operate in Martial Class by having the application running and opening an Attendance Window to select the required class (day and time).
You can then use your wedge scanner to record attendance by scanning Student IDs. This will only operate whilst the Attendance window is the active window on your machine.
This is essentially a "Manual" Attendance operation as you cannot configure a keyboard wedge to automatically switch which class it is recording attendance for.
Serial style scanners are monitored through the Full Contact background program (Regardless of whether you are using them in automatic or manual attendance modes). The main application need not be running for serial scanners to operate in automatic mode.