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 ) . It is sometimes abbreviated to cu m , m3 , M3 , m ^ 3 , m \*\* 3 , CBM , cbm when superscript characters or markup can not be used ( e.g. in some typewritten documents and postings in Usenet newsgroups ) .