A type definition [TD] is a custom control (CTL) file that has been marked as a type definition.
Instances of the TD are linked to their definition. You cannot edit functional properties of TD's instances, but only visual properties. To change functional properties you have to edit their TD. All changes in a TD are propagated to all of its instances, when you save the TD or choose to 'apply changes'.
Type definitions may be nested, i.e. may contain other TDs. You use this to define clusters of different data, which are defined as well and as such can be accessed on different levels. Review the properties of, say, a waveform chart, using a property node. You'll find a property 'Bounds' for the chart itself as well as for its caption, label, scales, even the scales' markers and so on.