use title content as existing field I have created content type with multiple entries some of them have term reference / autocomplete.

Why cant i do that to the title for the content type? 
Is there a way I can make that happen? 

Also why can't I use the title in other content types?

Attached image with title .
The reason you can't use the title as you wish, is because it's not a Field, but a Property.

Unfortunately I can't find a meaningful explanation of the difference between the two at this time, so no link, sorry.

If it helps in any way, you can create your own title field, which will then behave more like you expect. Of course, you'll then have two "titles", so this will various annoying side effects.