I have an oil fired hot water boiler with a tankless coil installed. Im considering adding an indirect hot water tank. Is that tank fed by the tankless coil in the boiler? Or is the tankless coil removed from the boiler, and opening sealed and covered with plate, and new tank fed with main hot output line from boiler, which also heats home? Thanks, Joe

You are messing with something best left to a professional.

Is that tank fed by the tankless coil in the boiler

Sometimes yes and sometimes no depending upon temp rise and flow rate.

Or is the tankless coil removed from the boiler, and opening sealed and covered with plate,

You should not have to do that even if you do not use it.

and new tank fed with main hot output line from boiler, which also heats home?

Yes but here is the rub. The system has to be designed in such a way as to allow the domestic hot water generation to have a main control over the hot water produced by the boiler other wise when real cold weather hits you many never have enough domestic hot water for showers etc.

This includes electrical valving and a control system. You might be able to find some wiring diagrams on the net from various manufactures of tanks designed to be feed by a boiler to produce domestic hot water. BTW the tanks and controls are very expensive and it is probably cheaper since you are on oil to just buy a electric water heater and forget about this project. Just trying to save you some $$ and grief.