They are individual variables of a trade or asset, and not necessarily related to TMFs. For example, a TradeVar can be used to store the value of a certain indicator at the moment of entering a trade, for statistical purposes. And an AssetVar could be used for counting the number of trades that were entered with that asset.

But the usual case is using TradeVars in a TMF for storing values that need to be preserved between TMF calls. For instance, counting something or storing the maximum or minimum of something.