explain what is "digital delayed surround" sound

I purchased from a garage sale a 10 out 10 looking SONY str av770x audio video control center, but no owners manual, think the unit is mid 1980s technology.. It has place for speakers A and B and another for rear speakers . What exactly are these rear speakers for . Is that used for suuround sound but the receiver has no 5.1 or 6.1 DTH etc. so really is this just simulated surround sound if I play a DVD movie with 5.1 it won`t really be surround sound right or what are these rear speakers for? Should I buy speakers for the rear?

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