A VSTi (VST Instrument ) is a VST Plugin that can convert MIDI events to audio. You can select a VSTi by clicking the instrument slot's down arrow. The VSTi plugins appear in the list's 'VSTi Plugins' section (Windows) / 'VSTi and AUi Plugins' section (Mac).
A VST or VSTi plugin will initially appear in both Effect and Instrument selectors. The first time it is loaded MultitrackStudio determines whether it's an effect and/or an instrument. From then on it will appear in the appropriate selector only.
VST Instruments look like VST Plugins. They have an additional Channel Selector. Some VSTi plugins respond to all MIDI channels the same way, in this case the channel setting doesn't matter unless you're recording using the Keyboard Splitter or multiple keyboards. Other plugins, especially the ones providing many different sounds, do always require the correct channel to be specified.
VST Instruments respond to all the streams that are in the track's MIDI file. However, in most cases it will be easier to use multiple tracks (each using an instance of the VSTi) instead.
If the plugin has more than two outputs an Output button will be available. On clicking this button the Output Mixer window will appear. This mixer mixes the plugin's outputs down to stereo.
Tip (Pro edition only): The outputs of a multiple output VSTi plugin can be routed to tracks using sidechaining (see External Sidechain Routing). You can use this if you'd rather mix the outputs in the main window rather than the VSTi Output Mixer window. This is especially useful for drum plugins.
If a VSTi plugin gets in a bad state somehow you can press the F5 key (Windows) / Command-R (Mac) while the plugin interface is visible. This will save the settings, reload the plugin and load the settings again. This doesn't work while the track is recording.