Good Morning -
What is the difference between grade_proctor, login_proctor and ta?
Would a ta or grade proctor be able to authenticate students for a proctored exam?
A login_proctor can only authenticate students to start a proctored exam, but can not authenticate students to grade a proctored exam at the end.
A grade_proctor can authenticate students to both start and grade a proctored exam.
A ta can also do both. In addition, a ta can do many other things like view unopened sets, view hidden sets, show answers before the due date, access instructor tools (although they can't do much with them), etc.
See the permissionLevels starting on line 751 of conf/defaults.config for more details.