The tax effects would need to be certain, I think you are right. They couldn't be guessing about any of that or they are asking for total loss.
I have read somewhere that a company establishes a valuation allowance (like an allowance for doubtful accounts) when management believes it is likely that some (or all) of a deferred tax asset will not be realized. This is how NOLs are carried on the balance sheet as assets-- deferred tax assets.
This is also complicated and needs to be totally understood by management.