Quote from tradelink:
I believe they are referring to fact that IB's data is sampled. (or many snapshots for data, rather than the continous stream of data).
A full time and sales stream means you get every trade, and every bid/ask update when the market is open...
but a snapshot means they are only giving you say every 5th trade, so you miss a lot of information.
If you have to ask the question it probably doesn't make a difference to you... but it's something to be aware of.