I assume, from the way you describe it, you’re referring to the “cue dot” that I referred to in my last post. As in this example:
Without repeating too much of what I said above, this “dot” was either generated live by the station playing out a show, or burnt into the tape itself (which is why they would also be seen in exported copies). On ITV and Channel 4, the cue dot was always somewhere in the top right. The intention was to inform the 14 regional companies that an opt point (normally the end of a segment or the very end of the show) was imminent so they would be ready to roll their ads/promos or cut to local presentation. Except in the case of live shows, the stations would already know this info as they would have a “routine sheet/presentation schedule” which showed every “event” of the day and its source, but it assisted the playout staff at each station nonetheless.
Here’s another example, again from ITV, where you see two cue dots:
Why two? Well, one was on the VT to start with, and the other was being added live. But why, you still ask? Well, because in this case the end of the program wasn’t the opt-out point. Something else was being networked after the show (such as a promo or announcement) that all regions were required to screen. The original dot would disappear 5 seconds before the end of the show (a useful cue in itself for the originating station to cut to the announcement/promo) but the second “live” dot would remain on screen until 5 seconds before the end of the announcement/promo - the real opt point and cue for the regions.
You can see what I mean here in this video - a “pub anno” follows the episode of Rainbow (nothing to do with alcohol! - it means “publication announcement” and promoted a book or video that accompanied the program just broadcast). So the other stations wouldn’t opt out until the anno had ended.
Credit: VideotapeFTW - YouTube
The flickering on the dot often got faster as the opt point approached - again probably to keep the regions on their toes.
Channel Four used cue dots like this prior to 1992 because their commercial breaks were played out by the ITV regions, even though there were no regional variations in schedule (except in Wales, who had a totally different fourth channel, S4C). So a dot would appear before each ad break.
The BBC used cue dots too - normally on the top left of the screen - as a cue for VT operators to run promos or programs or to change source. Often a BBC region would put a cue dot on their feed in order to check if they were “in circuit”, ie. whether they were locked onto the network feed from London, or whether they had “soft opted” which allowed them to add their own supers to the national feed, or indeed opt out properly for local news, programs or promos.
You don’t see these dots very often now - partly due to centralised playout and shared idents and presentation across most of ITV, but they are often still used in one form or another during live programs to advise presentation that an ad break is coming up.
If you see a cue dot appear and then disappear during a repeat of an old ITV show, it will mean there was a commercial break in the show that has been cut out. ABC runs them without breaks, of course, and the commercial channels will probably schedule their breaks at different points (you get more ad-breaks per hour in Australia!). Hope this helps!