In SCPI all messages that you can send to an instrument are divided into two categories: commands (that do not imply getting any response) and queries (that allow to get response back from the instrument). Queries have question mark ('?') at the end of command header.
A command or query is called a program message unit. A program message unit consists of a header followed by zero or more parameters:
<header> [parameter [,parameter ,...]]
For example, in the query
FETCH:ARRAY? MAX, A
FETCH:ARRAY is a header and
A are parameters.
One or more program message units (commands) may be sent within a simple program message:
<program message unit>; <program message unit>; <program message unit>...