This SELCAL app is a tone encoder which sounds SELCAL 12/16/32 tone selective calling tones.
The tones are produced in accordance with ICAO Annex 10 Volume II ARINC Characteristic 714-6 Mark 3 ARINC 714A Mark 4 and ASRI SELCAL specifications.

It can be of great use to test SELCAL decoders since you will always have it with you unlike those expensive and few SELCAL text boxes which are never around when you need them.