SF (Sequence Focus) - You do things a step at a time, even when you're doing 
more than one thing at a time, each you do a step at a time. And the things you 
do can be or include doing the nine VIC commands.

In Simple Terms:

The typical "grab the next line of the active sequenced-file, or interactive 
user input, process the line for VIC variables and commands, then pass it (or 
its return values) to the device, program, program-port, etc. as set by OP, when
there is something to pass on (not totally consumed by SF processing.) All this 
with optional user interactive editing during the steps of processing such input