In simple terms issues which are determined by the higher courts (in England the High Court and Court of Appeal, across UK including Scotland the Supreme Court amongst others) become binding precedent or authority binding lower courts and tribunals unless they are overturned by a higher court on appeal, or a higher or equivalent court otherwise says that is wrong or decides another case differently on the same point. Higher courts may also make statements about the law in relation to matters which are NOT issues requiring determination in their particular case. Those comments or guidance are known as obiter dicta, things said in passing. They are not binding, ie lower courts don’t have to follow them, but may be persuasive. How persuasive they are depends on things like whether they were fully argued, the seniority of the court saying them, etc. FWC was not a case in which the issue of trans people in changing rooms and single sex spaces needed to be decided, so what the SC said about that is obiter. The ET does not have to follow it, but it is highly persuasive as it was not a comment said in passing, it was a matter which the SC gave considerable thought to and addressed a number of paragraphs to, and the SC is the highest court in the land. Almost everything they say, arguably, is persuasive.
JR is going to say SC in FWS did not determine anything about single sex spaces, NC is going to say ok they did not but they have very clear guidance which is highly persuasive and you ET will have to have very good reasons for not following that guidance.
Hope that explanation helps.