The cdebconf progress bars are currently driven by the following protocol:

PROGRESS START <min> <max> <title-template>
PROGRESS SET <val>
PROGRESS STEP <inc>
PROGRESS INFO <info-template>
PROGRESS STOP

Proposed protocol, not implemented yet:

PROGRESS REGION <min> <max>
  - Mark a region of the progress bar which a subsequent (nested) PROGRESS
    START will occupy. Titles given to nested PROGRESS START commands will
    stack, and thus previous titles will be restored on PROGRESS STOP
    (details of display may depend on the frontend); PROGRESS SET and STEP
    commands within the sub-bar will be mapped linearly onto the main
    progress bar; info messages will not be erased when exiting a sub-bar
    and are expected to be superseded by future PROGRESS INFO commands.