The client submits to all relays that you have configured on your account to submit to ("write"). For example I have configured the relay.damus.io to be read and write and all others to read only because I don't want people to think I'm submitting to more relays than the damus one. I wish I could submit to more relays than one simultaneously on my CLI tool but at the moment it doesn't seem to let me do that, it only allows one at a time which is different from your experience on most web client and apps where you can select "write" access for all your relays if you want to.