The Bishop - diagonal moves
The white bishop on a1 has a clear diagonal from a1-h15 and can occupy any of the squares shown.
The white bishop on a8 has its path blocked by a piece of its own colour on c13 and so can only occupy the squares a9-c12.
The white bishop on e1 has its path blocked by a black pawn on g5. It can occupy squares f2-g4 or capture the pawn on g5.