Depends on your setup. If you have a AVR, and use bass management then the sub receives the LFE signal plus the <hz bass from the 7.0 channels, which is set below the xover point. Crossover on sub disabled. That's bass management.
If you have a stereo system, then use either left/right pre-out from the stereo amp, or speaker level L/R from the stereo amp. Use subs own crossover.
In pure direct, then the L/R are set to full range, the bass management is disabled, no signal from LFE is sent. So sub receives no signal. That's pure direct working fine.
If you use pure direct, and use high level the sub receives full range, you need to use the subs own crossover. Basically same as two channel hifi with sub. Which is fine. However the problem is when using in AV mode, the sub will be receving signal from RCA (LFE + other channel bass) and you want the subs own crossover disabled. Ok. However the sub will be receving L/R signal from the speaker level, you've bypassed the subs own crossover (because unlikely you will adjust dial every time) and the sub will be outputting LFE + 7.0 bass managed bass + L/R full range (and/or depending on sub works with crossover, ie if crossover is disabled in certain input priority) Left and right (trying to play) full range. So it'll be multiple combined bass, it'll be a freaking mess lol.
Unless of course you have a seperate AVR + stereo amp. That'll work fine in stereo mode (AVR off) but in av mode both will be on, and both be sending signal to the sub (high and low level)
Simply use the single RCA-RCA cable if you have a AVR. Use bass management- or not, and when not your sub will be quiet.
Rel are stuck from 25 years ago lol, they probably think bass management is bad, so you must use high level- but they must also please the av guys so "hey use both at the same time" derp. without thinking what happens.