There can been be various codes.
Like if one wants to start a text conversation and make sure the coast is clear, it could just contain a secret pre agreed word...like January.
Then the expected response could be Roger or Snow.
If there is no response, then the other party will not proceed with the conversation in case the husband or wife is around.
A coded response, also let's the AP know the coast isn't clear...there's not one set code, as it's unique to those in the affair.
Where the APs number is stored under a man's name, the starting message can be "are you watching the match"
Basically anything that wouldn't arouse suspicion if stumbled on.
Other codes I've heard of are a location name that is special to the couple.
Incriminating conversations will be deleted from the message trail..if they have any sense.