TOPCAT makes use of a tool interoperability protocol called PLASTIC (PLatform for AStronomical InterConnection). This can be used to exchange messages between TOPCAT and other PLASTIC-aware tools such as Aladin, VisIVO and STILTS. The messages which are relevant to TOPCAT are things like "load this table" or "highlight this set of rows".
The communication works by all tools communicating with a central 'hub' process, so a hub must be running in order for the messaging to operate. If a hub is running when TOPCAT starts, it will connect to it automatically, listening for messages sent by other tools. If not, you can start a hub from the Interop menu in the Control Window or start one externally. Other tools will have their own policy for connecting to the hub, but in general it's best to start a hub first before starting up the tools which you want to talk to it.
The interoperability has two aspects to it: on the one hand TOPCAT
can send messages to other applications which cause them to do
things, and on the other hand TOPCAT can receive and act on such
messages sent by other applications.
These are described separately in the subsections below.
There is also a section on the Interop
menu in the
Control Window used to control hub operations.
The PLASTIC protocol itself is still under development and may undergo changes in the future. It is therefore possible that there may be version compatibility issues which arise between TOPCAT and other PLASTIC-compliant tools and services, so these facilities should probably be regarded as experimental at present.