A format name must end in a suffix that indicates the type of format (except for the names of paragraph formats, which do not require a suffix). How a format configuration file is referenced in [Templates] settings determines which types of formats are valid in the referenced file, and thus the valid suffixes for the names of formats defined in that file; see §7.2 Working with format configuration files. Table 7-1 shows valid suffixes for format names, depending on the [Templates] reference.
DITA2Go validates formats partly by suffix. A wrong type could result in very odd output, plus a warning in the log file.