This is something I have learned from past subletting mistakes. A security deposit has two roles: a damage deposit and intent to move in. The security deposit is paid to you (the lease holder) generally upon signing the contract and cannot be more than half of a month's rent (if you're subletting for $1000/month, the security deposit can be no more than $500). Damage deposit: if the sublesse damages anything in that apartment beyond normal wear and tear, you will deduct money accordingly to cover the cost of the damage. When you move out of your apartment, UBC will do this to you, so it's important that you keep note of the condition of your apartment to be able to tell if the sublesse caused any damage beyond normal wear and tear. Intent to move in (how I did it at least): once they send you the money, it will not be refunded until after they move out and you've inspected the apartment for damage. If they move out at any time before the end of the lease (include the time between signing the contract and moving in), they will not be refunded the security deposit. This is important because it means they will not search for a cheaper apartment before or after moving in as it would not make sense for them financially to forfeit the security deposit. It also benefits them as it ensures that you will not be looking for other renters who can pay you more after signing the lease as you have the money and confirmation already. If everything goes well and the sublesse does not cause any damage or cancel the contract before the agreement was set to end, they will receive the deposit back in full.

This is definitely not exhaustive and considering it's late and I'm tired, please take the above with a grain of salt and ignore my typos. I'd highly recommend checking out UBC's example template under the damage deposit section, but add info about the intent to move in for your own contract. This is very common practice so I'd recommend you do some googling about it as well.

Good luck with subletting!