Some commands require extra information to specify their operation. For example, the Move (m) command requires the specification of the destination line address:
{line1},{line2}m{line3}
Other commands like the Zap (z) command represent a class of commands which are specified by sub-command characters. For example:
zcd
is a zap cursor delete command. The cd is a subcommand of the zap command and will not be interpreted as the major commands c and d. This form of subcommand is used by several editor major commands.