SAMP stands for Simple Application Messaging Protocol. It is a protocol which allows tools on the desktop to communicate with each other. Very briefly, the way it works is that applications can send messages to a central hub process which will then pass them on to other applications which can respond to them in some appropriate way. GAIA can use SAMP to send and receive images, sky position information, and catalogues or identification of rows within them. It can also send extracted spectra. You will require a SAMP hub, either free-standing or integral to some other application, to use these capabilities.
Applications which currently talk SAMP include TOPCAT, SPLAT and DS9, amongst others.
You can find more information about how it works, what applications are compatible, and how it can be useful at the SAMP web page, http://www.ivoa.net/samp/.
In order for SAMP communications to work, a SAMP hub must be running on your machine. This is not part of GAIA and you will have to start it up separately. Several other applications include their own hubs, which may be started automatically or by user request; these include the AstroGrid Desktop, TOPCAT and Aladin. Alternatively you can run a freestanding hub such as JSAMP or SAMPy. See the SAMP web page for links to these.
If the hub is running when GAIA starts up, GAIA will connect to it ("register") automatically. If you start the hub while GAIA is already running, you can connect to it by selecting the "Register" option from GAIA's "Interop" menu. When registered, GAIA will listen out for messages from other applications and act on them. You will also be able to send messages to other applications. If you want to stop GAIA responding to any such messages, you can select "Unregister" from the same menu.
In most cases, sending a SAMP message from GAIA is controlled using one of its "Interop" menus. For a given action these often have two ways to transmit a given message: "Broadcast ..." and "Send to ...". For "Broadcast", all registered applications which understand it are invited to do something with the message. For "Send to" there is a submenu which allows you to choose which single application to direct the message to. Only applications which claim to understand the particular message (in the jargon; which subscribe to the relevant MType) will appear on such submenus; this means that the "Send to ..." submenu may be empty, depending on what other SAMP-aware tools you've got running.
GAIA can both send and respond to messages in three general application areas: image-related, catalogue-related and spectrum-related. The messages and how to use them are listed below; the MTypes (strings in parentheses) are the formal identifiers for these messages - if you are not an applications programmer you can ignore these.
PLASTIC support has been withdrawn from GAIA. Use SAMP instead.