Thanks for your interest! "Container" here isn't related to any programming concept, it's just how I decided to name some parts of the algorithm... in the code, they must be simply some JS objects.
I'm in fact a bit ashamed of how primitive the code in The Master of the Land. I think you can import it into Twine and see the actual Snowman code, but I'm afraid it's really unreadable. I didn't write the game with Snowman, but with a custom pseudocode that I converted to JS using the AWK utility. All extremely complicated.