Taxonomy term reference field only from top level terms of a vocabulary I have a multilevel taxonomy term vocabulary 


Degrees and
i. Specializations


I'd like to use it in two content types. In one case only the top level terms should appear for users, while in other case users can choose only from the 2nd level (this one can be solved by Hierarchical Select module). 

Is there any extension of taxonomy form elements that can limit the level that  users can choose from?
If you replace your taxonomy term reference field with an Entity reference field you will be able to use the Reference field option limit module to restrict the available options for dependant fields. If you need to apply further restrictions you can also generate your options lists via special entity reference views.

Note however that by switching the field type to entity reference you will no longer be able to use taxonomy term specific fields and filters for those fields in views.