Sure.
Reg-T margin follows the rules set by the American regulators for margin lending on small accounts. There are tight restrictions on leverage - generally only up to 2x - and there are restrictions on what products you can trade. But if you've got less than $100k of equity in your account, it's the only option available.
Portfolio margin is more generous with leverage - you can usually leverage up to 5x or 6x depending on the product - and no restrictions on what products you can trade, but you have to have at least $100k in your account to be able to switch portfolio margin on.
Thanks again for the speedy explanation!