Indicating in 42 is playing a specific domino to tell your partner something about your hand.
Is this cheating?
Lets look at a few examples:
My partner's trumps are 6's and he leads the 66.
Since I have 22 00 11 25 40 43 50 in my hand, I decide to play the 22.
Most partners will interpret this to mean that I have the next highest 2 in my hand which in this case is the 25 since 26 would be a trump. There is a good possibility that my partner will have 21 or 20 and can lead them to me to get me in the lead.
If you say that this is cheating, you are telling me that I can only play certain dominoes in my hand based on what my other dominoes are. I can't see anything in the rules of 42 that would give you that right. If I want to play a 22 and I am not reneging, that is my right.
The key to this is, I did not have to tell my partner prior to the game that if I play the 22 it means I have the next highest 2. I can learn whether a partner indicates his/her doubles just by watching their play over the course of a game. Because all hands are not played out fully on the 42-online site, this process can take longer. I see nothing wrong with asking a prospective partner if they indicate prior to game since that is something I can learn over time.
The above logic works just as well for another example:
Again, my partner's trumps are 6's and I have none.
Since I have 44 55 32 54 40 11 10 I decide to play the 40.
This tells my partner that I have the next highest 4 which in this case is the 44. While this is not as obvious, you cannot call this cheating, because to do so, would limit me to playing a low domino from a suit for which I have no double.
What if my hand were 44 55 33 40 50 32 31
According to those who say indicating is cheating, I have no legal dominoes to play in this hand.