1. Holding deposit is to hold the unit for the potential tenant BEFORE he/she moves in.
Seucirty deposit is to cover damages caused by the tenant(s) AFTER he/she moves in.
2. Please google holding deposit and see how it is carried out.
Now you should sign a holding depoist agreement, pay holding deposit. and usually on the move in date, you and the tenant sign lease/pay rent/exchange key.
Holding deposit agreement says Holding deposit will be transfered towards the balance Security deposit at lease signing/rent paying/key exchange meeting.