Action only
A particular rule in poker requires that in a game, only a full or half bet can be re-raised, with respect to an all-in bet. What this means is that when anything other than a full or half bet is made, it is considered as action only, and now all the remaining players can only call the bet. They are not allowed to re-raise the bet anymore.
Another rule in most card rooms is that only a full bet is considered as legitimate. Anything less than a full bet will then be considered as action only. An example of this is, suppose Player A bets $100, following which Player B calls. Now Player C decides to go all in for just $20. When the bet moves back to Player A, s/he can only call not raise the bet as now the situation is action only.
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