Collateral is deposited and withdrawn as SPL-USDC. All perpetual markets are quoted against USDC and are linear perpetual swaps. Perpetuals on Drift Protocol are cross-margined against each other, meaning that the profits and losses from a single position can be used to cover PnL of multiple other positions in the same user account.
Currently, to ensure sufficient compute for on-chain liquidations, the maximum number of positions a user can have in a cross margined account is five (5). (Note that this max includes both currently held positions and markets that have pending orders).
Initial/partial/maintenace margin requirements: see Liquidations and Contract Specs
account health: 0 - 100%. (at ~0% subject to full liquidaiton)
Trading with leverage puts you at risk for liquidations. Holding a position through enough funding payment intervals (which require payment) also puts you at risk.
(see Liquidations for more information)