PING

An easy way to see if your transmissions are successfully making their way to the OTA bot, and to find out which path they took to get there.

Whatever arguments you give to the PING command will be returned to you in the PONG response.

Which path you came in on

heard on RF means your packet was transmitted on a radio and an iGate put it on APRS-IS, which is what rule #3 requires of a host. The other answer is heard on APRS-IS:

That means your packet reached the bot over the internet and no radio was involved. PING itself is happy either way, so this is a successful test and not a refusal. It matters when you want to host an op, because a host’s messages have to come in on RF. If you expected heard on RF and got this instead, the usual cause is your app rather than your radio: see point your app at your radio, not the internet.

This is worth doing before you go on the air rather than after. One PING tells you what a HOST would tell you, without spending your op on it.

Reference