Basically a Sa spiral arms are more tightly bound than a Sb. A Sc has very loose spiral arms.
See related link for a pictorial
Stereo System =SC-AK770 Main unit = SA-AK770 Front speakers = SB-AK770 Subwoofer = SB-WAK770 In short, the SA-AK770 would be a stereo with no speakers or subwoofer.
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NGC 3486 is a type Sb spiral galaxy.
uzuki engine versus Briggs and Stratton
SB and SA are two types of galaxies which are described respectively as barred and normal. Both types fall within the spiral form which galaxies may assume. S and SA more specifically qualify for the classifications of ordinary or unbarred within the spiral form of normal galaxies while only SB receives the categorization of barred type of normal, spiral galaxy.