For all ICs. They are small reservoirs of energy for when the IC needs it fast.
Unless the datasheet says otherwise, use one 100nF between gnd and each power rail used by the IC.
That means that cmos logic ICs for example just need one between +12V and gnd as they don’t use -12V (usually… I’m sure somebody comes up with cases where they are powered between +12V and -12V and not +12V and gnd…).
They are a cent or two each and can’t hurt anyway…
Power regulators need their own combination of decoupling caps…
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