With a drainable pouch, you need a way to keep it closed when you’re not emptying. Most manufacturers offer two types of closure systems:
External (a clip that closes off the bag)
Attached (velcro that attaches to itself to close the bag, but each company has their own unique name for this)
The velcro can be less noticeable under clothes but in my experience, more messy when emptying. If you have a habit of dropping things, the clip might not be for you since it comes separate from your pouch when you empty (kind of like the gas cap of a car that isn’t connected) so you have to make sure you put it in a safe place and don’t drop it in the toilet. It’s advised to always carry an extra clip on you if you use this kind of closure.
If you have hand dexterity struggles, try them both and pick the one that’s easier. Some of the velcro systems require some good old fashioned squeezing to make sure they’re fully connected. Some clips are easier than others, depending on the brand.