query related to Teaching Assistantship award
Most places run on one year budget cycles so a commitment beyond one year would be hard to assure. But unless the educational system falls apart and there are no students to teach, getting a renewal would be assumed. Of course you need to do a good job also to get a continuation of a TA. But research institutions couldn't really run without lots of TAs in some fields.

But they make a "promise" to provide funding. You can expect best efforts on their part but it isn't an absolute guarantee. 

In the US, in particular, State Universities are subject to budget constraints made by the legislature. They just have to deal with it.