Any Rosegarden track assigned to an audio or synth plugin instrument becomes part of Rosegarden's audio routing architecture. There are 16 audio instruments and 16 synth plugin instruments, and each of these has a set of adjustable parameters including fader level, pan, effects and so on. These are generally configurable in the Instrument Parameter Box or audio mixer. Each audio instrument also has an input connection, and both audio and synth plugin instruments have an output connection.
An instrument's input connection defines where Rosegarden will read audio data from when recording to this instrument. The most usual setting would be to one of Rosegarden's external audio inputs, the number of which is configurable in the audio mixer. These correspond to JACK audio ports exposed by Rosegarden, to which you can connect outputs from any other JACK audio application or soundcard capture ports using an external JACK router application such as QJackCtl. You can also set an instrument's audio input to the output of one of Rosegarden's built-in audio submasters or master output for mixdowns.
An instrument's output connection defines where the audio goes when played. Instruments default to sending their audio to Rosegarden's master output, which is another (stereo pair of) external JACK port(s) that can then be connected up using the same external JACK router. However, Rosegarden also supports up to eight internal submaster busses, to which you can send instrument outputs instead so as to group instruments into related categories. You can control the number and routing of these in the audio mixer.
Rosegarden currently supports effects using a set of pre-fader insert effects slots for each audio instrument. You cannot yet assign effects to the submaster busses, and Rosegarden does not yet support send effects, although you can obtain similar results using an external JACK effects and mixing architecture.