Custom tags don't invoke tag files.
Tag files are the *special case* of tags that don't need a tag class.
Tag files are an easy way for your non-java-programming team to include
EL-processed
jsp fragment files, like headers or footers.
Custom tags, alternatively, require a
java class extended by one of the Tag Support classes, and thus require a nice meaty section in your TLD file.
DaveS
SCJP