Assuming that you have not applied any Layer Effects to a layer, most blend modes behave the same way when you adjust the Opacity or Fill. But there are 8 blend modes that behave differently when Opacity is adjusted, compared to when Fill is adjusted. Meaning that with these 8 blend modes, 50% Opacity will look different compared to 50% Fill.
These blend modes are Color Burn, Linear Burn, Color Dodge, Linear Dodge (Add), Vivid Light, Linear Light, Hard Mix, and Difference.
Also, check out my video on Blend Modes.
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