As long as the defence is facing the offence, not moving forward, and is standing in an upright position then the defence has a legal guarding position.
If the defence moves any part of their body to the side without moving the feet or forwards into the offence then it is a block.
A charge should be called if the contact is in the centre of the body, ie between the nipples.
A charge can be called while the defence is in the air as long as they had a legal guarding position and have not moved forward.
It MAY be left as a no call if the offence gained no advantage and the defence was not disadvantaged.
Same goes for screens.