This depends whether it is an internal or external application.
If external, then Pilkington Pyroclear® is the inner pane. This is based on the presumption that the greatest risk comes from fire inside the building and that the objective is primarily compartmentation to keep the fire in its place of origin. That should be a major consideration, for example, to prevent flame break out and minimise the risk of fire movement to facing buildings or up the outside of a high rise from floor to floor.
If internal, then the orientation of the unit should be determined based on the most likely direction for protection against fire as part of the risk assessment. In those situations Pilkington Pyroclear® may be used in conjunction with another toughened, laminated or coated glass as the second pane.
Note that application, as always, is governed by the scope of test evidence.