An imposition template can force the use of a particular display box when the imposition is activated. Afterwards the used display box can be changed by the user as desired.
The settings for the individual display box types govern the automated creation of those boxes on the imposed sheets. The boxes can be off, they can be set to a pre-specified margin (manual size), or they can be calculated according to another source:
The crop box can be derived from the sheet margins defined in the document settings, or overridden in the template itself. The bleed box and the trim box can be derived from the bleed mark and from the trim mark settings in the document settings.
The art box can be derived from the source page in one up layouts. This setting has no effect on more complex layouts.