Why can not copy License Key?
It is selectable, but not editable field, and of course, you are allowed to copy its value. The way to select the value is either using the mouse, selecting the text from start to the end. Usually you can also select the entire field using triple left-click. Once the text is selected, you can copy it using CTRL+C, or right click on your mouse and select Copy.
I am asking that what we have functionality from DLM then what is difference between DLM from you add-on ?
We did not compare our add-on to any other addon when we created it. We created it based on features we and some our customers needed, that contacted us asking to create such add-on, and we did our best creating the add-on. If you are interested in similarities and differences with other add-ons, feel free to check add-on descriptions and compare them.