This is used in dialogue result fields to ask a decision of the player or can be called just to "continue" a longer speech. After the PC makes his choice, the same topic will be checked again, and you can provide the correct response by using function / choice / = / choice_enum in the speaker conditions of the dialogue window. Choice can be used in script, if the dialogue window is open. If the same resultbox/script contains more than one choice call, the choices are presented to the player as a single list (unlike MessageBoxes).
The limit of choices per result or per call may well be version dependent. It has been reported that there is a limit of 5 choices per call, but I'm not sure which version this was tested with. Under version 1.6.1820, I don't think there's a limit on the number of choice calls in a single result, but only 20 choices can be displayed on screen at one time: if more are displayed, the game will freeze when the player clicks one (this applies to scripts as well as dialogue results). More than 20 possible choices won't cause problems as long as no more than 20 are actually displayed (i.e. using conditional statements to select 20 or less from a larger number of choices is OK).
On using Choice in script: I don't advise using it in a script that also contains a StartScript command, as this can also freeze the game sometimes. If you need to use both, you might try delaying the StartScript part until out of menumode if possible or using some condition to ensure that the choices can't be given more than once (not tested).
