I want to start a registry for my sets ,and my questions are:
As far as I recall-
1) NGC coins cannot be part of a competitive registry set, but you can add them to your personal "Inventory" spreadsheet and can include them in non-competitive "Showcase" sets
2) There should be a link to the PCGS registry on the PCGS homepage https://www.pcgs.com
3) Yes, a coin can be part of multiple sets
No NGC coins cannot be part of any set competitive or showcase (the same as the NGC custom set). You can add them to your inventory only.
I am not aware of any set definitions, however if you choose a set from the list of available sets in the registry there will be a blue box in the upper right that says set composition. If you click that it will display all the coins needed plus some scoring info like the weight for each coin and any bonuses if applicable.
Yes coins can be used in multiple competitive and showcase sets simultaneously.
I am not aware of any PCGS registry options that permit you to publicly show /display your NGC coins. I have included NGC pieces in my Inventory but do not see any way to add those to a Showcases (Nor Registry).
Unfortunately for our host - If you like to combine and display your entire collection - NGC registry is a better option as they permit PCGS coins in their registry side by side with NGC grade peices. In addition they have a custom set option that you can also create your own unique set if a specific NGC registry is not defined. Their custom sets also lets you combine NGC and PCGS graded coins and they can viewed by other collectors .
It is apparent that I was wrong about the third part of the first answer I gave. I don't do much with the registry, but had erroneously thought you could display NGC pieces in a showcase.
Thanx to all for advice
Welcome to the boards.
Good luck with your set.