I have query that, why for comp flow we are using mass flow at inlet and for external flow we can use velo at inlet? pls reply Thanks

Maybe because in a steady flow computation you eventually want to have mass in = mass out for the internal case, and for an external flow case we don't have that as a hard and fast restriction (i.e. mass can leave along multiple parts of the farfield boundary), and the flow at the boundary can adjust to account for global conservation.

Also note that typically these are among the easiest and most natural quantities to define at those particular boundaries from a problem definition perspective.