One thing a professional union does is build walls around itself so that it knows who it’s representing. These walls are usually based on agreed qualifications to do the job, so interlopers are not getting in that will lower the status of the union and its members or weaken their bargaining power. (Hence, Nemesis's question in the other thread is such a pertinent one.)
Another thing is ideally to reach agreement with the employer or the employers’ representative body to deal only with the one union.
Having attained agreement on who can be a member, the union can then work out what services it provides and at what cost to the members.
Having gained recognition from the employer/s it can then negotiate with them on salaries, terms and conditions.
If there is a dispute, the union will represent members, but only up to a point. If it is established that the member is at fault, the union will back off. After all, it needs to be seen as responsible and consistent.
Where the member is guilty of dereliction of duty, the union will not back him/her indefinitely. Where the employer is guilty, the union and employer will sit down to negotiate. If agreement can’t be reached industrial action will be taken until it can. This tests the strength of both parties. Agreement of some kind is nearly always reached and binding on members of both unions and employer organizations. Excessive militancy is usually pulled into line by political action or weakened by fragmentation of the militant body.
TEFL and related teachers in Thailand have no agreed criteria for membership of the profession. It clearly isn’t a profession for many. And who would a TEFL teachers’ union negotiate with? Many are in the private sector, with no real sense of fraternity, and it’s hard to see private employers cooperating to the point they’ll sit down with unions in the gentleman’s club that industrial relations becomes in the West. The whole face thing would prevent any kind of negotiation. I suspect that, in Thailand, employers would give up on English teaching before they'd concede to industrial bargaining. They'd diversify or even lose money rather than lose face.