It was a reply to the OP stating they were considering spending time and effort installing a two pair cable.

But what the OP says is

The extension run is about 45ft and the router (VDSL 80/20Mbps) will be plugged into the extension.

No mention of an SSFP.

If this is a ‘normal’ extension (ie just across 5 and 2 of the standard faceplate) then no matter where the router was connected, either end, that would be a bridge tap.

My understanding of a bridge tap is an *unterminated* pair branched off the main line. This acts as a transmission line to the high frequencies involved in VDSL. The energy goes up the branch and is reflected back at the open end. (It has to go somewhere...) The waves coming back interfere with the waves going up and either cancel each other out or reinforce each other and/or shift phase depending on the frequency and the length of the cable. This interferes with discrete frequencies in the VDSL signal. If the branch is terminated by a modem, this has the right impedance to encourage the energy to continue into the modem in the proper way.

The unterminated branch can also act as an antenna and pick up radio interference.

A suitable terminating resistance, e.g. 600 ohms, could alternatively be used to absorb the energy.

There is another context for 'bridge tap' and this is where a single pair in the distribution network is branched and laid to multiple distribution points for possible future use. I don't know if this was done in UK, but it was in US, where the term may have originated.

There is another context for 'bridge tap' and this is where a single pair in the distribution network is branched and laid to multiple distribution points for possible future use. I don't know if this was done in UK, but it was in US, where the term may have originated.

Correct, and no, doesn’t happen as a rule here.

Don’t agree with the unterminated pair bit.

I disagree, if the router is at the far end of the extension where is the bridge tap?

I would concur with @thinker27 who describes my understanding of a bridge tap.

Your interpretation seems to be different, please explain why you have a different take on the situation.

A TDR would probably see the terminated socket with nothing plugged in, which means it's affecting the signal. It's by no means as much of an issue as star wiring but it depends what sort of SnR headroom you already have on your service.

What do you mean by an terminated socket with noting plugged in? I think the far end of the extension will have a modem/router and possibly a telephone connected.

My understanding is that the incoming line is going into the Openreach NTE and then an unfiltered extension is being installed for the modem to connect to. This NTE will likely show up on a TDR if you went looking for it but I can't see it being a significant event on a trace unless your line is truly marginal.

If you don't also want a phone at the master socket location and you're on an underground feed I'd be tempted to run a new cable through to where the existing underground cable is gel crimped onto the cable that runs indoors and relocate the master. Technically not something you should DIY but it doesn't really matter if you don't screw it up.

This NTE will likely show up on a TDR if you went looking for it but I can't see it being a significant event on a trace unless your line is truly marginal.

If testing at the extension will show a steady stream of of FEC errors… add the bell wire, more still. All these things make for a slightly less happy socket.

Your interpretation seems to be different, please explain why you have a different take on the situation.

20+ years hands on experience faulting these circuits in a myriad different scenarios.