Axapta uses quite a number of file extensions for the files that are installed as part of Axapta. Even though at first sight, people will be bewildered with these te(o)ns acronyms/extensions, it follows a very good naming convention and once it’s  understood, it is very easy to follow these file names and extensions.

First Letter: owner of file:

A Application
K Kernel

Second Letter: contents of file:

L Label
O Object (as is AOT)
T Text (only TKD)
D Developer Documentation
H Online Help

Third Letter: Type of file.
D Data
I Index
C Cache
T Temporary

So a file with extension .ald is ApplicationLabelData file.

Thanks to Michael Fruergaard Pontoppidan (who came out this table) and Tao Wang (who provided this info to me)

In theory there could be 2x5x4 = 40 file extensions :)