When making entity selections in AutoCAD, sometimes the results can be unexpected. You may select a hatch object and move it its associative boundary stays put, or you may erase a line and other geometry disappears with it. Situations like this can be controlled by a system varible named PICKSTYLE.
Many moons ago in the AutoCAD world, when you received a command prompt instead of an expected dialog box, you could check the system variables FILEDIA and CMDDIA and make sure they were set to 1. Over the years, CMDDIA has all but been phased out in AutoCAD, although FILEDIA (for FILE DIAlogs) does still apply. Here is a related post on this.
PICKADD is one of those system variables that you see mentioned a lot, usually in a negative light. Somehow it gets set to 0 and then you can only select one object at a time. Who would ever want to do this, right? Although 99% of the time, you probably keep PICKADD set to 1, there is a useful reason to set it to 0. If you are editing or viewing properties of multiple entities one at a time, this can be helpful.
If PICKADD = 1 and you select one entity to view its properties in the Properties window, no problem. But what happens when you select a second entity? Of course, Properties displays the properties for both entities. You won’t get much useful info if you select say a SPLINE and an MTEXT entity. To avoid this, you have to press ESCAPE to cancel the previous selection each time.
Now, set PICKADD to 0 and then each time you pick an entity, only that one entity is selected and you can edit or view its properties.
If this is something you think might help you, consider using the following lisp code to toggle the value of PICKADD from 1 to 0 to 1 and so on.
The system variable PICKADD has been changed for 2011. Most people probably will not notice, but I like it. Previously, there were 2 choices for this variable, 0 and 1. A setting of 2 was added. See the summary below.
0 - means that only your last selection is valid. In other words, if you select a entity, then select another entity, the first one becomes unselected.
1 - is what most people use. This allows you to continue to add entities to a selection set.
2 - works just like 1, expect that when using the ._Select command, the entities remain selected when the command is over. Previously when you used the ._Select command then executed a command, you had to use _P to refer to the previous selection set. Now you can use the ._Select command, create a selection and then launch the desired command with the selected objects pre-selected.
Have you ever set a system variable and then you are working along and that sysvar is changed to a different value? Many people will ask how or why it "changed itself", which obviously it can't do. Of course what is really going on is a program, whether it be an internally defined command, or a lisp/VBA/ARX routine, has changed it. Read more about System Variables changing on their own?