from
https://en.wikipedia.org/wiki/Betting_in_poker
Incomplete bet or raise
If a player goes all in with a bet or raise rather than a call, another special rule comes into play. There are two options in common use: pot-limit and no-limit games usually use what is called the full bet rule, while fixed-limit and spread-limit games may use either the full bet rule or the half bet rule. The full bet rule states that if the amount of an all-in bet is less than the minimum bet, or if the amount of an all-in raise is less than the full amount of the previous raise, it does not constitute a "real" raise, and therefore does not reopen the betting action. The half bet rule states that if an all-in bet or raise is equal to or larger than half the minimum amount, it does constitute a raise and reopens the action.
For example, with the full bet rule in effect, a player opens the betting round for $20, and the next player has a total stake of $30. They may raise to $30, declaring themselves all in, but this does not constitute a "real" raise, in the following sense: if a third player now calls the $30, and the first player's turn to act comes up, they may now call the additional $10, but they do not have the right to re-raise further. The all-in player's pseudo-raise was really just a call with some extra money, and the third player's call was just a call, so the initial opener's bet was simply called by both remaining players, closing the betting round (even though they must still equalize the money by putting in the additional $10). If the half bet rule were being used, then that raise would count as a genuine raise and the first player would be entitled to re-raise if they chose to (creating a side pot for the amount of their re-raise and the third player's call, if any).
In a game with a half bet rule, a player may complete an incomplete raise, if that player still has the right to raise (in other words, if that player has not yet acted in the betting round, or has not yet acted since the last full bet or raise). The act of completing a bet or raise reopens the betting to other remaining opponents.
For example, four players are in a hand, playing with a limit betting structure and a half bet rule. The current betting round is $20. Alice checks, and Dianne checks. Carol goes all-in for $5. Joane, still to act, has the following options: fold, call $5, or complete the bet to a total of $20. If Joane calls the $5, Alice and Dianne only have the option of calling or folding; neither can raise. But if Joane completes, either of them could raise.