MSSL (the Mathematics and Statistics Software Library) is a collection of (mostly Fortran) routines, in source code form, that is made available to LC users via the LINMath website. Source code downloaded from LINMath can then be moved to any other desired LC machine.
Historically, MSSL was divided into three classes according to the level of quality control and support. Class 1 and 2 routines required quality assurance, at least minimal support, full usage documentation, and some testing. The Class 3 routines were minimally screened but usage-documented. Following the demise of LC's MSS, these distinctions lost much of their meaning. However, the Class 3 routines are still kept separate, in a collection called mssl3 (about 300 routines), while the name MSSL now refers to the collection of all Class 1 and 2 routines (about 100 routines). Many routines in mssl3 may well be of high quality but simply never went through the screening and testing necessary to raise their class level.
There is one particular set of routines in mssl3 worth highlighting: a trio of routines in the Service Routines category—I0MACH, R0MACH, and D0MACH—that provides various machine constants. The results they return are often more accurate than the values in the corresponding MSSL or SLATEC routines. For details, see the section Largest and Smallest Numbers.
There is no separate MSSL or mssl3 reference manual. The usage documentation for each MSSL or mssl3 routine is provided in the comment line prologue of the source file that is delivered by LINMath.