Difference between modules in KEGG Module
M00115 is a pathway module (NAD biosynthesis) whereas M00542 is a signature module (EHEC/EPEC pathogenicity signature).

From the KEGG page on modules:


  
  pathway modules – representing tight functional units in KEGG
  metabolic pathway maps, such as M00002 (Glycolysis, core module
  involving three-carbon compounds)
  signature modules – as markers of phenotypes, such as M00363 (EHEC
  pathogenicity signature, Shiga toxin)