If I open the User / Client Variables screen and mouse over the Client1 field the helper text popup says "This is represented by the variable {client1}". These helper text popups don't properly align with the actual variable names.
Helper: client1, Actual: clientcustom1, clc1
Helper: client2, Actual: clientcustom2, clc2
Helper: client3, Actual: clientcustom3, clc3
Helper: user1, Actual: usercustom1, urc1
Helper: user2, Actual: usercustom2, urc2
Helper: user3, Actual: usercustom3, urc3