If you are sent on a casting from your agency, always ASK Questions.
Ask what it is for before you show up clueless - Is it a print ad, commercial, ad campaign,etc.
If this is hard to get out of the agent or random casting director them be a little concerned. If an agent or casting director is sending you to a casting THEY should know these details.
Also ask, how much it pays, and what it involves.
It is best to get this up front so you wont end up confused, or uncomfortable at the casting and also so you know how much the job is paying ahead of time,in case you book it.
These days my agencies give me castings sometimes over email which is nice because it is all laid out, Name of the place, product, brand, casting, Time, place, rate, etc. Also it is good to keep it on file, eitehr written or email your self the information so you have it handy.