Independent reading levels allow students to apply comprehension strategies in books at their reach. Teachers confer with students on their independent books to progress monitor and formatively assess.
Guided, or instructional, reading levels allow students to read a level above their independent level - with guidance from the teacher, typically in small groups.
Frustration levels are above a student's grasp and are not appropriate for students to read independently. They do not allow students to apply comprehension strategies. They are appropriate for interactive read alouds, during a small percentage of a literacy block or at another time during the school day, since students can listen at a higher reading level than they can read.