What is MarketData response type?

In the 1.3.1 beta there is a new response type called "MarketData". (I noticed because a compiler error check tells me I am not covering all possible values in a switch.) There is already a "MarketDataSnapshot" type, so I'm wondering:

• what the difference is


• if this is a response I want to be paying attention to


• and if so how do I do that (none of the cpp samples appear to be using it).

Thanks!