Data String Contents Byte Definition Standard Enhanced 1 1 A 1-character presentation space short name (PSID). 2–4 Reserved. 2 5 One of the following characters: L to lock the API. U to unlock the API. 3 6 One of the following characters: R to return if the presentation space is already locked by an application. Q to queue the Lock request if the presentation space is already locked by an application. 7–8 Reserved.