First, here's the table with all the boring text and numbers. I've added colour to spice it up a little bit, but deliberately chose pastel colours because I don't want to overstimulate you.
You can see from the table above that there are five channel planning blocks, each with six channels. That's so that ABC, SBS, the Seven Network, the Nine Network, and Network Ten all have space to broadcast at each location, along with a spare channel. Ostensibly, that spare channel is for community TV, but it could be another commercial broadcaster, emergency broadcasts, or even to help DAB+ spread Australia-wide.
When trying to think about digital TV channel restacking all those years ago, the decision was made to create a shorthand way of talking about groups of channels. The idea was that when you've got multiple TV transmitters/towers that are geographically close and might overlap, you don't want these to be on the same frequencies.
Take the lovely Victorian country town of Colbinabbin, home of some stunning silo artwork: https://www.countrynews.com.au/news/2020/05/18/1177915/colbinabbins-silo-art-is-a-celebration-of-the-towns-history
It's also almost equidistant from three TV transmitters:
- Seymour - Channels 40-44 on UHF V
- Bendigo - Channels 28-33 on UHF IV
- Goulburn Valley - Channels 34-38 on UHF IV and UHF V
Or, in Channel Planning Block terms:
- Seymour - Block D
- Bendigo - Block B
- Goulburn Valley - Block C
Extrapolate this across all of Australia, and you've got a quick and easy way to make sure that there are no channel block overlaps. It's the same way you plan out larger WiFi access point systems while dealing with limited 2.4GHz or 5GHz channels, just FYI.
By the way, Colbinabbin is by no means unique in having the potential for overlapping transmitters. Have a look at Newcastle! Given the terrain up there, there's less overlap than you'd think from this image, but I know the TV antenna installers in the area have a lot of rogue signals to deal with.
Thinking in channel blocks makes it easy to filter out anything except your local channels by using a band-pass filter.
For Colbinabbin, if you wanted the channels from Seymour, but the Goulburn Valley channels were just a little too strong or a local LTE (~700MHz) phone signal was causing problems, no problem. Just buy a Block D band-pass filter (like this one from Aerial Industries, or this one from Matchmaster) and the only thing left will be your Seymour channels.
In the Newcastle example, those TV antenna installers often carry a range of band-pass filters in their vans and use them liberally to create clean signals for their clients.
These band-pass filters are perfect for removing rogue signals before you use a masthead amplifier to boost the overall levels. Why boost noise if you don't have to?
Our range of TV frequency filters is here. Some will filter anything outside of a single block like in the examples above. Others will filter all of the blocks to remove signals other than TV. And some are just there to drop any frequency above 700MHz to get rid of often very-powerful LTE phone signals. Also in that product category are attenuators (dropping signal levels when they're too strong) and baluns (converting from one type of signal to another).
One last thing - you'll notice that Channels 9 and 9A aren't assigned to a block. This is because they're primarily used for DAB+ radio in the capital cities, for now at least. A couple of places use the upper part of Channel 8 to broadcast DAB+ too. There is always the possibility that these might be used for future digital TV broadcasts when we get sick of the TV stations we have now!