"..., when I add a text area, MATLAB gives the name TextArea by default to the obejct and I am not able to change it."
Ah, but you can change it (or any other control) to whatever you want. Select the control you want and use F2 and it will let you rename the control. It then becomes app.MySpecificTextArea and you're all set...
You can find out all about using App Designer in the doc tutorial, <the specific subsection> explains all about naming/labels including customizing them.
I never built a GUI using GUIDE (I don't do GUIs in general) so I don't know anything at all about how it identified its controls programmatically, but with AppDesigner, the Tag property is just that; user data associated with the particular control but has nothing to do with its name for identification programmatically.
