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In the world of analog closed captioning, there was caption channel 1,2,3 4, and Text channels 1 and 2. All of them were required to have data related to the specific program being aired at the time. So although Text could display up to 15 lines of 32 characters each, it couldn't be advertisements for the local car wash. Program information was fine, though.
I don't think Teletext had that limitation, though I never saw it in action stateside.
There was another vertical-interval data service called, I think, Wink TV. It would broadcast URL links (within commercials) that could be embedded somewhere in Line21 caption data (Possibly XDS, station information zone). It was to be recovered with some sort of set-top box which would then display website info, either from the station, cable, or modem -- don't remember which. I recall this because some of my caption software could embed these links. Never had a customer with the need.
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