I need help understanding "crystallization" for this discussion.

Computation of deferred taxes under IFRS is slightly different from GAAP. For example, in the United Kingdom (which follows IFRS), companies use the crystallization approach. An equivalent concept in the United States is "realization."

The concept underlying this "crystallization" approach is that companies recognize deferred income taxes only if the taxes are expected to crystallize. Therefore, if a liability is deferred indefinitely, then the present value of that liability is zero. No deferred tax liability is recognized if the accumulated deferred tax amount is expected to increase each year, thereby delaying indefinitely the ultimate liquidation of this obligation.

I need help in understanding the difference between "crystallization" and "inter period allocation" used in the US.