Self ID is used in two different ways, and they're both common. Actually, one is more common than the other, but when it is useful for those posting gender> sex, 'misunderstanding'' is standard and useful, as seen on the Twitter thread in OP.
The more common use of self ID is the social rather than legal side.
About large numbers of sports bodies, changing facilities including in gyms etc, bogs natch, police crime recording, etc.
Going on self ID of gender (internal feeling) rather than sex. And loads of other things. Rape crisis, women only competitions, awards, feminist orgs, I mean so so much.
Often not publicised, pointed out when joining, using facilities etc. Which is so cowardly.
Anyway. That self ID, what's demanded is that everyone assume everyone is in the right place. Even a double take or other slight reaction is vv upsetting. Saying something is hateful. Just keep quiet, ignore or better still be welcoming (!). Remember appearance doesn't mean anything about what gender, and gender can be fluid. Ground floor seeing same person women's changing, later use gents. So what? Be respectful. Their gender could be in flux and don't judge...
Etc etc.
Second is law. GRC, if move to self ID.
Very few trans people here have one, proportionately.
This changes birth cert. Also marriage cert iirc if married (whole so called 'spousal veto'). Legally your sex has changed.
Where helpful?
For full on old school trans sexual people, means no confusion thought was women this says male.
Not sure what else have to look.
If was like Ireland eg online form.
What issue with it.
With so much self ID already not sure.
Ireland Barbie K quickly did it before in court so their law said women's prison.
Dunno really. Again would need to look.