If you are in the US, the same phone has different submodels for each US operator, and some of these submodels (likely from AT&T and Verizon) may have a locked

If you are in the US, the same phone has different submodels for each US operator, and some of these submodels (likely from AT&T and Verizon) may have a locked bootloader, preventing you from installing custom ROMs. For example, Samsung Galaxy S5 from T-Mobile (SM-G900T) you can put Cyanogenmod on, but Samsumg Galaxy S5 from AT&T (SM-G900A) you can not.