A format tag represents the name for a format type. A filter tag represents the name for a filter type. Typically, an ACM driver supports one or more types of formats and/or filters. Within Windows NT DDK documentation, ACM data structures, and sample ACM drivers, the term tag is used more often than type.
Typically, a set of formats or filters is associated with each tag. For example, the sample IMA ADPCM audio codec supports two format tags, namely the WAVE_FORMAT_PCM tag for the PCM format type and the WAVE_FORMAT_IMA_ADPCM tag for the IMA ADPCM format type. For each of these format tags, the driver supports several formats consisting of various sample rates and sample sizes.
To differentiate between codecs and converters, we can say that a codec transforms data from a format belonging to one format tag into a format belonging to another format tag, while a converter transforms data from one format to another belonging to the same format tag.
Format tags and filter tags are defined in mmreg.h. If you are writing a driver for a new format type or filter type, you must register the type by using the Multimedia Developer Registration Kit.