They are physically and offically one platform but for passenger easy they are called A and B for platforms 7 to 15. This is to permit passengers to be at the right end of the platform for short train such a five coach IET but when there is a long train such as a nine coach 345 then they just say the platform number and do not say platform end A and B. For example a nine coach 345 on platform next to the Down Relief Loop (platform 13), the staff would say platform 13 but if it was just a five coach IET depending on where it stops it could be Platform 13, Platform 13 A or Platform 13 B. https://sacuksprodnrdigital0001.blob.core.windows.net/sectional-appendix/Sectional%20Appendix%20full%20PDFs%20Sept%2023/Western%20&%20Wales%20Sectional%20Appendix%20September%202023.pdf