NGC Grade PL Vs Unciculated Set Issue MS

This is the difference per NGC and applies also to canadian and other 'world' coins.

Most circulation issue coins have minimally reflective fields, even in Mint State (MS) grades. When a circulation issue coin's fields exhibit a higher-than-normal degree of reflectivity, the coin is said to be "Prooflike" (or "PL") because it is reminiscent of a Proof coin. If the fields are deeply mirrored, the coin is said to be "Deep Prooflike" (or "DPL").

NGC uses the PL or DPL modifiers after an MS coin's numeric grade, when applicable. These designations indicate only the degree of reflectivity evident in a coin's fields and make no implication as to other qualities associated with Proof coins, such as frosted devices and superior sharpness.

https://www.ngccoin.com/news/articl...p-prooflike/

All I can find so far is that Uncirculated Set Issues are a Collection of Brilliant Uncirculated Coins from 1968 to 2010 as described in the Charlton Catalogue of Canadian Coins v.2. These brilliant uncirculated coin sets were a continuation of the Proof-Like sets issues only issued in nickel rather than silver. In 2011 the mint discontinued the brilliant uncirculated (Charlton v.2, 4th Edition, Pg. 338) finish and issued circulation strike coins in sets.

So that may be why 1967 you see PL and 1968+ you can see the Uncirculated Set issues MS (Mint State or graded with a grade)
https://coins.www.collectors-societ...aspx?s=12321

Thank you so much to everyone for your help and advise.

There are some findings after deep diving into lot of catalogs and list of auctions.

It looks like all coins from Canadian mint sets between 1954 and 1967 were graded as PL and they are continue grading as PL category. At least I didn't find any coin before 1968 graded as "Uncirculated Set Issue MS".

It was some grace period (between 1968 and 1978) when coins from mint sets were continue graded as PL. But new category/grade "Uncirculated Set Issue MS" was used for coins from those mint sets too.
It looks like that NGC uses PL and "Uncirculated Set Issue MS" grades as interchangeable terms. Coins graded as PL could be added into uncirculated NGC competitive sets and opposite.
Currently all coins from mint sets since 1968 are graded as "Uncirculated Set Issue MS".