Just checked the SynScan Manual and the actual wording is :
Auto Guide Speed -
When using an autoguider, this sets the guiding speed to 1X, 0.75X,0.5X, 0.25X, or 0.125X sidereal rate.
So ignore unless guiding.
Here are the other rates available with explanations from the manual:
SET TRACKING
Sid. Rate: This activates tracking in Sidereal rate (R.A. Tracking).
Lunar Rate: This activates tracking in Lunar rate (R.A. Tracking).
Solar Rate: This activates tracking in Solar rate (R.A. Tracking).
PEC + Sidereal Rate: Sidereal rate with Periodic Error Compensation.
Stop Tracking: This stops the tracking instantly.