checksum

hello dears，
First of all ，I am a beginner of SmartServo。If I ask some easy questions ，please don't mind。I find a example program in user's guide。The code is as below。

SADDR1
ECHO_OFF
PID4
KP=100
KD=500
F
A=1600
V=4000000
MP
mm=1 //Initialize Minus Flag to one
ss=0 //Initialize Main Checksum
tt=0 //Initialize Command-Only Checksum
nn=0 //Initialize Input Number to Zero
yy=0 //Initialize Incoming Checksum Flag
....
....
.....
My question is that how to find out the meaning of the variables，such as mm，ss。 Does every variable have a certain meaning？How can i know their function？Is there a diagram using to describe them？And I am also confused by Checksum。What is the algorithm of the checksum？
If you have anything about Date Variables Memory Map， please tell me。Thanks in advance。