Select all (keyboard shortcut ⌘A) does just what you would expect and selects all the visible objects on a drawing. I does not select objects on non-visible layers. And it can only select objects on visible layers other than the active layer if "Selection on all visible layers" option is checked in the Window >> Layers/Pages... pallet.
Special select... brings up the window shown right. This allows you to choose what to select and how they should be selected. This can be a very powerful tool, especially in a complex drawing.
Objects: Choose which objects you wish to select. The default is All. The moment you deselect anything, it will change to Some. Clicking None will deselect all the objects and then once you select one, or more, it will change back to Some.
Attributes: Choose the attributes of the objects to be selected.
For example to select all Lines on your drawing that are coloured Red, you would select "Line" in the Objects column and "Pen color" in the Attributes column. Once you have selected "Pen color", the color pallet becomes active by clicking the little colour box, and selecting the colour of the lines you wish to select.
All objects. When you hit the OK button, this will select all visible Red Lines on your drawing. If the red lines are on different visible layers, it will select them provided "Selection on all visible layers" is checked.
With mouse. When you hit the OK button, nothing will be selected until you draw a marquee with the mouse around the objects you wish to select. In our example, only Red Lines within the marquee will be selected; Red Lines outside the marquee will not be selected.
Deselect all before. Generally if an object is selected and you draw a marquee around it and other objects, the selected object is deselected and the remainder selected. "Deselect all before" deselects any objects that were selected before you chose "Special select ...", so that they get included in the selection process
Add to selection. This allows you to add a second (or more) set of objects to your selection. So, for example as well as wanting to select Red Lines, you also wanted to select Blue Ovals, you could first select the Red lines as outlined above and then, without clicking the mouse on the drawing (which would deselect the Red Lines), return to the "Special select ..." menu and choose Blue Ovals and "Add to selection". Hit OK and you will find that you have, quite logically, now selected Red Lines and Blue Ovals.
Tip: Squares are selected as Rectangles and Circles as Ovals. Groups will select all groups, regardless of the attributes – because groups can have mixed attributes.
Length object = 0 or empty string:
Checking this box will select "empty" objects – that is objects that have no length or objects whose position data is Not a Number (NaN). These are perhaps fragments left over from a trimming operation of some sort. I have found the most common to be arcs presumably left over from a failed radius fillet trim. This can result in an arc where the Edit pallet shows xc = NAN, yc = NAN, and probably L= 0; or it might simply show AA = 0 and L= 0 as either of the images to the right.
The presence of these NaN or zero length arcs will cause RealCADD to crash – often to the extent that you can't even start to draw a marquee anywhere on the drawing with any of the select tools, without causing a crash. Empty lines don't seem to cause a crash or at least I haven't found that to be the case.
If this is happening to you, the first thing to do is to check for empty objects. I usually click "None" in the Objects menu and then check the "Arc" box. Check the "Length object = 0 or empty string" box. Check the "All Objects" box. Hit OK. If there are any empty arcs showing in the Edit pallet, simply click Delete or ⌘X to get rid of them.
I usually repeat this with Line – uncheck the Arc box and check the Line box to see if you have any empty or zero length lines. Delete them if you have. You can try other objects, though so far I have only found arcs and lines. I find it best to try one object at a time, rather than multiple objects.
Doing this will usually solve any constant crash problems that you might be getting