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dcap is decoupling capacitance, capa between vdd and vss.
Tap cell is to add std cells connection to substrate.
Decap is dcap.
There is for std cell library also some endcap, to be place first and last std cells of each rows.
decap are generally used to increase the capacitance over VDD and VSS lines in order to decrease its ripple when high current is withdrawn from the supplies while clock is toggling.
Tap cells are used to spread the substrate potential around the area to keep the Vt levels as predicted.