Question for the electronic pixie wranglers

I'm trying to understand the differences between the following findings in a scientific paper.

Constant Potential of −0.20 V

Dense aluminium electrodeposits can be obtained on both W and Al electrodes using constant potential deposition.

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VS

Constant Current 50mA

Dense aluminium electrodeposits cannot be obtained using constant current deposition from AlCl3?TMPAC ionic liquids at various current densities (50?200 mA/cm2) and temperatures (60 and 90 ?C).

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I get the CC  but have no idea what Constant Potential of −0.20 V means.

Any pointers please?