Fishing reel - line capacity?

I was wondering what the numbers 6lb/xxx yds 8lb/xx yds mean
I guess that means line capacity right?
I've read that using a heaver weighted line can damage your reel?
I was wondering if there's anything bad about using a lesser weighted line? Like 4lb line for a 6lb reel?
Update: My question was about using a lower weighted line for the reel

like 4lb line for 6lb reel
is it bad for the reel? or maybe you lose sensitivity?
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