Taking 11 units usually implies that you are taking an Extension subject, although it could be a non-Extension 1 unit subject, such as Studies of Religion I as seen in your case. The rule is that only the best 10 units (which must include your best 2 units of English) will count towards your ATAR.
In the event where you have not performed well in one of your 2 unit subjects, SOR I can be used to reduce the negative effect that unfavourable performance in the 2 unit subject can have on your ATAR (provided you have performed well, or at least at a higher level in SOR I compared to the 2 unit subject). This is done by having only one unit count from the 2 unit subject, allowing for SOR I, being also one unit, to count towards your ATAR, which halves the negative effect that the 2 unit subject would have initially had on your ATAR.
If SOR I ends up being your lowest-performing subject, then it will simply not count towards your ATAR. Instead, the remaining 10 units (which will be comprised of your 2 unit subjects) will contribute towards your ATAR.
I hope this helps!
