Complex sentence: Once the applicant has signed the contract, their place at the medical school is tied to the MRBS scheme, which means they can't decide to change their place from MRBS to CSP/BMP.
Split into simpler sentences: Once the applicant has signed the contract, their place at the medical school is tied to the MRBS scheme. Which means they can't decide to change their place from MRBS to CSP/BMP.