I lower load a DTD.zip from Pubmed central,a database special in Medicine. It's not a Dtd file,it includes folders,many dtd files,many ing files .
the dtd file: ftp://ftp.ncbi.nih.gov/pub/archive_dtd/publishing/
What you are seeing is really a modularized DTD. The DTD consists of a variety of modules (usually .ing files or .mod files) to ensure that bits of the DTD could be used again or overlooked/incorporated easily.
These different modules are usually recommended from inside basics DTD by utilizing parameter organizations.
For instance, an XML instance will still reference just one DTD with it's DOCTYPE declaration, but that DTD may have parameter entity terms/references with other files which contain terms for element, attlist, entity, etc.
Using "journal-posting-dtd-3..zip" out of your link as the second example, you might have an XML instance that validates to "journalpublishing3.dtd", but when you see that DTD you'll notice that you will find references to two other modules ("journalpubcustom-modules3.ing" and "modules3.ing") which themselves reference much more modules.