Advance Subscription Agreement — Trying to Understand How It Works

I keep coming across the term advance subscription agreement while researching early stage fundraising options in the UK, and wanted to actually understand how it works before assuming it is the same thing as a SAFE with a different name. Here is what I have pieced together so far, would appreciate anyone correcting me or adding context.

An advance subscription agreement is an investment that converts into shares at a future date or event, usually the next priced funding round, without setting a valuation at the time the money is invested. It is not a loan, there is no interest charged and no repayment obligation, which seems to be the main reason UK startups tend to prefer it over a convertible loan note. It also seems to work better than a SAFE for UK companies specifically, since a SAFE was designed around US company law.

One detail that seems important is the longstop date, a maximum period after which the investment must convert into shares regardless of whether a priced round has happened, which exists partly to satisfy HMRC requirements around SEIS and EIS tax relief.

What I am still trying to understand is how founders typically decide between an advance subscription agreement and a full priced equity round at a very early stage.

Has anyone here actually raised using an advance subscription agreement in the UK? Curious how the terms compared to what you expected, and whether the longstop date or conversion terms ever became a point of negotiation.

Found a fairly clear explanation of this on Entrepreneur Plus while I was reading around the topic, helped me understand the founder side of it a bit better.