As usual they have great difficulty splitting out the idea of sex and gender, and which words they're using for which
Sex: physical features including genes, hormones, internal organs and external features. Not binary.
Saying this, but completely failing to mention intersex at any point. I think this could be explained better - eg. Vast majority of people are male or female, some people have genetic or hormonal differences that are described as intersex.
Sex: assigned at birth and recorded on birth certificate. This 'legal sex' is rarely relevant. Currently binary in the UK. Changed by applying to Gender Recognition Panel.
Given the above, they should dump all this 'assigned' nonsense - sex at birth is fine, and as to legal sex rarely being relevant, how do they square that with their previous statement - assuming that they really mean sex not gender here:
Gender is important in UNISON. We use gender monitoring and rules on proportionality to address historic, institutional discrimination against women and women's under-representation