Different Puppy versions come with different software built-in. That is software that is pre-installed in the base .sfs file. For example in Puppy525 the base file is lupu_525.sfs this file contains the operating system files, window manager etc AND some built-in software.
Usually it is lightweight programs are pre-installed to keep the filesize down. For example, mtPaint, InkLite, Gnumeric and AbiWord.