Capacitors using a plastic foil as dielectric come in two flavours: With real metal foils as electrodes, or with a metalization evaporated directly on the plastic foil. The latter consumes much less volume, so higher cpacitances are made of metalized plastic foils. The caps with real metal foils are much larger but offer considerable less drift.

I don't think that there's a standard ruling the labeling of plastic foil caps. But there's a traditionally grown labeling. It's as follows:

In German "plastic foil" means "Kunststoff-Folie". Take the "K" of this word. (Remember, that some very traditional cap manufacturers were in Germany when the plastic foil cap was invented (WIMA, Roederstein, Siemens, etc.).)

If you use a metalized plastic foil, then take "M" for "metalized", which is in German "metalisiert".

If the foil is made of polyester, then take "T" as abreviation. So, you get "MKT" for a cap using a metalized polyester foil.
By the way, polyster caps are perhaps better known as "mylar caps".

If you have a cap using real metal foils instead of a metalization at the plastic foil, then omit the "M". Then you get "KT".

If the plastic foil is made of polycarbonat, then take "C", instead of "T". If the foil is made of polypropylen, then take a "P". So, a cap using a metalized polycarbonat foil is called "MKC", using a metallized polypropylen foil is called "MKP".
Using real metal foils instead of metalization gives "KC", respectively "KP".

The number behind this abbreviation does not follow any standard, at least to my knowlegde. I think it's an arbitrary labeling of manufacturer.

Caps from the company WIMA are called differently: Polyester caps get a "S" instead of "T". So, a cap using a metalized polyester foil is called "MKS". If real metal foils are used instead of a metalization, then the cap is called "FKS", because of the German word "Folie" for "metal foils".

You talked about a "MKT1.68 equivalent" in your last post. This makes you believe, that there's a standard called "MKT1.68", right? But there isn't such a standard, at least to my knowledge. I think it means something different: To be "MKT1.68 equivalent" shall state, that the cap follows same specifications as an earlier cap series of same manufacturer, which was called "MKT1.68" and which has become obsolete in the meantime.

Kai