Hello.
I would like to experience absolute happiness in my miserable life. 
So, I've created few actions and in some cases it requires to select
all tracks - Ctrl+A key press - before applying these actions.
I looked over help and forum and didn't find how it can be done.
It would be nice to put/program "Select All tracks" action in the action list.
I have found "_ALL" action, but it seems apply to all tags in all tracks.
Could you provide any info on the topic please?
There is currently no action that selects all files. The only way is either the keyboard shortcut or the Edit menu.
I have some more questions:
Should it overwrite the current selection?
How should it act if there is no file selected - usually you cannot trigger any actions if you have not selected at least one file ...
What happens if not all files were selected prior to starting that action group ...
I do not think that such an action would be more practical than the Ctrl-A function.
=Should it overwrite the current selection?
When I open files or whole folder there is now selection. I have to do do Ctrl-a.
If there was something selected, then - yes - it should cancel that selection and select all files
in the grid. No actions should apply to files. We are speaking only about Ctrl-a key press.
=How should it act if there is no file selected
No matter what happened before - it should just select all files and no farther actions.
No, I did not mean selection of one or few files. I meant function which would do the same thing
as ctrl-a. For example, if I open all files for one album and their track numbers are not numeric, but
something like 01/10, 02/10 and so on and I need to convert ALL of them to numeric format.
So I have to press ctrl-a every time I open new album. Or situations when I need to set disk
number, or rename filenames...
For example, I created few actions:
Disc 1 - sets 1 to disc number tag
Track int - converts non-numeric track number to numeric.
Format filename - overwrites existing filename with desired format
Every time I want to apply this changes to files I have to do following:
Open files in mp3tag.
Press CTRL-A on keyboard
Click Actions
Click OK
If I'd have CTRL-A function in action types or functions then
I 'd include it as first step in action group and would be like this:
Open files in mp3tag.
Click Actions
Click OK
Thank you.
I found a German thread dating back to 2003 that requests the automatic selection of all files:
/t/640/1
You can see what happened in the past 12 years ....
I do understand the philosophy of "at first select the target object and then execute an action against the selection" as the basic operating instruction for all modern computer user interfaces.
Because the action to be performed is dependent on the participating object, so the properties of the object determine the allowable actions.
Therefore an application can only be active and do something if it knows which objects are involved.
Only by the objects involved and their published properties it is defined, which actions can be performed on the objects.
You can also recognize this principle in the user dialog menus of Mp3tag.
The target object itself defines the permitted actions, not vice versa.
DD.20151106.1150.CET
If you put the three actions into one actionsgroup, then you can invoke the whole process like ...
DD.20151106.1219.CET
I very very seldom read only files in MP3Tag that should be mareked with CTRL-A.
There is no need to seperately handle folder per folder.
I do not want mp3tag to execute Ctrl-A in any stage. I just want it to be added as one
more action type, and then it would be up to user use it or not. That's all.
In this list : 
Is there a way to program custom action?
In the dialog window for "Action Groups" there is a "New" button in the button list at the right side of the window, just start from there to create your selfmade group/s of actions.
DD.20151106.1951.CET
I have thousands of albums to organize and function "Select All Tracks" would be perfect.
My suggestion is that in "Options Dialogue" in "General" be added to "When open Select All Tracks" option. So when loading the album, all tracks would be selected avoiding Ctrl + A key press.