What are subpackages?

Some articles on subpackages, subpackage:

SLATEC - Contents
... in SLATEC is tagged as belonging to one of 13 subpackages ... Some of these subpackages are also well known as free-standing FORTRAN subprogram libraries, including BLAS, EISPACK, FFTPACK, LINPACK and QUADPACK ... The following table shows all subpackages and the number of subroutines they contain subpackage number of routines separately available in Netlib purpose BLAS 114 ...