>>8096484i think "casual" means doing a more natural pose that looks like you're just minding your own business. imagine a naked lady just walking down a path, ignoring the camera man, even though she knows the guy is taking a photo.
the opposite of that would be doing a pose specially design to make the photo more attention grabbing or aesthetically pleasing. imagine a naked lady, doing an upside hand stand, while intentionally spreading her legs to flash her vagina to the camera guy.
but doing the 1st doesn't necessarily mean you can't do also the 2nd. it is possible to do both, but the 2nd is would be really hard to make look natural.
here is an example of a pose that is NOT natural/casual
