The following is considered a guideline used by the Wolff for how Non-Playing Characters (NPCs) are to be handled. The Rules on NPC's can be found here.
Non Playing Character's (NPC's) are the background characters that exist aboard the Wolff to flesh out the crew. Much like the random actor's walking the halls of the ship while the camera focuses on the main characters, NPC's exist in the RPG world to provide color and life to our ship. The Wolff classes our NPC's into one of three categories: Regular, Listed, and Protected NPC's.
Regular NPC's are pretty much the Red Shirts of the NPC world. Not listed on the simm's manifest, they may exist once in a post to say a single line or do a single action and that's it. They could be that nameless Red Shirt who get's killed every week. If they're in your department, create them as you need them, feel free to kill them off as well. If they're from another department and you just need a short interaction (say a security officer beaming and speaking with a transporter operator) go for it as well. If the interaction will impact the other department somehow, best to speak with the Department Head.
These are NPC's that are listed on the ship's manifest as an NPC. Good example of this from Star Trek TV series' are Adm Ross, Morn, Reg Barclay, etc. They're there and around but not used every single episode. Same idea here. We want them as recurring background characters but not regular use. On the Wolff I typically expect the Department Head takes responsibility for the characters so if you want to use the character, speak with the Department Head first. If it's a brief use, ask the Department Head and use them in your post. If you're going to use the character more then briefly in your post, offer the Department Head the option to play their NPC. You get a better post out of it and both of you get credit for the work.
These are special NPC's owned by a player on the simm. Typically PNPC's represent family members or characters the owning player wants to keep very tight creative control over. If you wish to use one of these players, you must speak with the PNPC's owner first, and they need to maintain creative control over the character. Good idea, regardless of how short the piece they're in, is to get the owner to write the part's unless they specifically okay you to use the character as you see fit.