Originally Posted by
seigex
Might not mean SSN, I just assumed so. I guess I'll find out if I get kicked back

but if it's optional, then maybe not a big deal.
I left it blank (in Canada the equivalent of an SSN would be our SIN) and my visa was issued.
Frankly I wouldn't share my SSN/SIN with anyone other than the IRS/CRA or a bank that I'm applying for a loan/credit card from, so the thought that that's what they were asking for never even crossed my mind.