|ALTNETS are uses of the AX-25 tocall to distinguish specialized traffic that may be flowing on the APRS-IS, but that are not intended to be part of normal APRS distribution to all normal APRS software operating in normal (default) modes. Proper APRS software that honors this design are supposed to IGNORE all ALTNETS unless the particular operator has selected an ALTNET to monitor for.
An example is when testing; an author may want to transmit objects all over his map for on-air testing, but does not want these to clutter everyone's maps or databases. He could use the ALTNET of "TEST" and client APRS software that respects the ALTNET concept should ignore these packets.
An ALTNET is defined to be ANY AX.25 TOCALL that is NOT one of the normal APRS TOCALL's. The normal TOCALL's that APRS is supposed to process are: ALL, BEACON, CQ, QST, GPSxxx and of course APxxxx.