Original: It is sometimes abbreviated to cu m, m3, m^3 or m**3 when superscript characters or markup cannot be used (i.e. in some typewritten documents and postings in Usenet newsgroups).
Simplified: It is sometimes abbreviated cu m, m3, m^3 or m**3 when superscript characters or markup are not available/accessible (i.e. in some typewritten documents and postings in Usenet newsgroups).