You can welcome applications from any group. So for example, you can welcome applications from members of the Asian community without it being discrimination, even though that does not include all other ethnic minorities and Asian descent is not specified in the Act.
That's true - but it's not the same. Race is a protected characteristic, so you can specify that you want applications from a particular racial or ethnic group.
Similarly, sex is a protected characteristic, so you can specify that you want applications from biological women - or indeed from biological men, if the role requires it (eg maybe it's a men's sexual health role).
Gender reassignment is also a protected characteristic, so you can specify that you want applications from people who have undergone, or are undergoing, gender reassignment.
What you can't do is say you want applications from some biological women, but not all (ie trans men), and some people with gender reassignment but not all (again, trans men).