Many operations require you to select entities before activating the relevant tool. For example, to reposition an entity you must select it and then activate the Move tool. You can only select entities that are in the current drawing layer and, more specifically, only entities that are in the current block.
There are several entity selection tools that you can use, each accessed from the Edit > Select menu. You can select entities by picking them individually, by surrounding them with a selection box or by intersecting them with a line. There are also tools for selecting and deselecting all entities. In addition, you can create a selection expression based upon entity type (line, arc, text, and so on), palette, block name, entity size and so on.
Using The Shift And Control Keys
By default, when picking to select entities, any previously selected entities are deselected. However, you can change this effect by holding down one or both of the Shift and Control keys when you make your pick:
Shift causes the picked entity to be selected
Control causes the selection of the picked entity to be toggled
Shift and Control together causes the picked entity to be unselected
In all three cases, any already selected entities are unchanged.
Selection Expressions
For many of the Selection Tools, the Command pull-down list appears on the Edit Bar and contains all your Selection Expressions (saved using the Select Expression Tool or via the Master Tool Settings). To use one of these Selection Expressions, choose the name of the expression you wish to use and press either the Add, Subtract or Retain buttons. See Select > Select Expression for more information. (If you do not see the Command pull-down list, it means you have not yet saved any Selection Expressions.)
The Selection Set
In precise terminology, when you select an entity, it is added to the selection set. The selection set is the collection of entities within the current block that have been selected and are currently visible. All entities within the selection set are always displayed in the selection set colour (configurable via Options > Project, Colours). When you change to a different current block (either directly, or by changing your current drawing layer) then your selection set changes. Entities that were displayed in the selection set colour revert to their usual colour and style. The selection set becomes those entities that were selected (and remain visible) the last time you worked on this block. If a selected entity is made invisible (for example by changing settings within View > Visibility), it is still selected, but it is removed from the selection set. Operations, such as the Delete Entity tool or the Transform > Move tool, only affect the selection set; they do not operate on invisible entities. When it is made visible again, the entity automatically becomes part of the selection set again and is displayed in the selection set colour.