Forcefield of Tok'ra design, capable of being configured to let in objects outside the field radius but not out again. The forcefield is usually cubical in shape, and a different form of energy shield technology than the Goa'uld force shield. The energy walls are erected using one or more forcefield generators.
The energy is not capable of containing the enhanced armor of a Kull warrior.
Evolution, Part 1 - SG-1 learns that a super-soldier is not confined by the power of the Tok'ra forcefield.