(*) secdef relates to securities definitions, such as contract details for all instruments (e.g. stocks, futures, etc)
(*) hmds stands for historical data management service. These are data farms that can deliver historical price data. They have this for USA, Europe, Asia.
(*) market data farms provide live market data (i.e. streaming market data). They use multiple of those, for various categories of instruments (e.g. US futures, options, forex)
(*) my guess is that with Primary they define the server which holds your account information.