Shimano HG41 v HG51 Cassette

Hi, does anyone know the difference in these 8 speed cassettes? Noticed some places state the HG51 is for a 'narrow' chain but i thought an 8 speed chain was just an 8 speed chain? The new chain i have is a KMC X8.99, my old KMC chain says narrow on the links but this new one doesn't
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both will work fine.
Parktools :?:SheldonBrown
HG51
HG41