married sectors are sectors who have availability as a through booking and so depending on availabilities may not be able to be changed separately

e.g. Passenger wishes to travel LGW-DXB-SIN in O class

Checking LGW-DXB, there may be no O class available but when checking LGW-SIN, the system may show O class available in conjunction with the onward flight to SIN. That O class availability LGW-DXB is maried to the DXB-SIN sector

If then wanting to change the DXB-SIN sector to a later date, this may not be possible since the O class on LGW-DXB is reliant on the DXB-SIN booking