Callouts are processed in this mode to produce the callout marks (or “bugs”) used to indicate cross references between the callouts and their corresponding areas. This mode should only produce the mark, for example “①”. See m:callout-link.

By default, callouts are numbered sequentially from 1 within each listing. However, a db processing instruction placed in the areaspec can specify a different starting number. If the starting-callout-number pseudo-attribute is an integer, that’s used as the starting number. If it has the special value “continues”, then numbering continues sequentially after the last callout number used in the preceding verbatim listing.