basedtech mark1 Stock rom is the official rom from manufacturer dats follow come rom
While custom rom is a customized rom designed to work on you fone, e.g samsung stock rom is customize to work on infinix, this i possibly bcos d rom was customize. So flashing of custom rom cn nt be flashed via dis method.

Things u should considered is know the exact spec of the device u want to upgrade nd download it right firmware.

And on no account should sp tool be interrupted when flashing is in progress.