To my mind a 'casting' is where you offer payment - cash or TFP - and a concept, even if it's just regarding levels.
"Photographer seeks slim, athletic model, 18-32yrs with no enhancements for 4-hr art-nude shooting, payment £150+ travel" is a casting.
"Model seeks photographer to shoot new beauty head-shots for online portfolio & Model-Book, MUA supplied, willing to pay up to £50 plus limited use of images - applicants must have proven track record in this style" is a casting.
However, if you're just touting for work, that isn't a casting but an 'availability notice' - you're simply announcing that you are available for work and reminding people of your rates...
"I'm looking for work for next month, see my rates, usual levels up to XXX" is NOT a casting.
"Due to a late cancellation, I have a 2-hour slot to fill on my studio day on Saturday, £40ph up to nude" is NOT a casting
Perhaps if enough people agree with me on this fine point of detail,
James could create an additional category and that way we might actually see some real castings on the Home Page and not just which models are available and wanting some extra cash this month...