Many tools within Impact use modifier keys (either Shift or Ctrl) to change the behaviour of the tool as they are getting mouse-click points from the user.
For example, most of the selection tools use Shift to add entities to the selection set and Ctrl to toggle the selection of entities. This is fully described in the Selecting Entities article.
When a tool is asking the user to identify an entity, any valid modifiers will be described in the status bar at the bottom of the window. If you press one of the modifier keys, that modifier description will be removed from the message.
In the above example, the status bar would initially read: Select an entity, Shift: Append selection, Control: Toggle selection. However, if you press the Shift key, the message becomes: Select an entity, Control: Toggle selection. but if you press the Ctrl key it becomes: Select an entity, Shift: Append selection.
Geometry Tools with the Constraint Manager
Tools that have special behaviour when the Constraint Manager is active also make extensive use of the modifier keys. This is documented for each of the appropriate tools.
For example, when using the Multiple Lines and Arcs tool, Impact attempts to infer for constraints. For example, it will attempt to match the length of an existing line, draw a line parallel to an existing line, make the new line horizontal etc. However, you can use the Ctrl modifier to prevent the tool from inferring constraints.
In this example, the status bar would initially read: Pick another point, Control: Don't infer constraints. However, if you press the Ctrl key, the message becomes: Pick another point.
Dynamic Constraint Tools
Most of the Dynamic Constraint tools make use of the Shift modifier. This is typically used to force the tool to keep adding more entities to the constraint being created. Again, this is documented for each of the appropriate tools.
For example, when using the Add Coincident Constraint tool, you would normally just identify two entities/points. But, by holding down the Shift key you can keep adding more entities/points to be included in the same constraint.
In this example, the status bar would initially read: Select a point, line or arc, Shift: Don't create constraints. However, if you press the Shift key, the message becomes: Select a point, line or arc.