Learn Italian: Qualsiasi ragione tu abbia, vale questo limite assoluto. - Whatever reason you have, this absolute limit applies.

The word qualsiasi (whatever/any) introduces a concessive clause. In Italian, these clauses express an idea of 'no matter what' and they always trigger the subjunctive mood. Since the subject is 'you', we use the present subjunctive abbia.

Could I use qualunque instead of qualsiasi in this sentence?

Yes, absolutely! When used as adjectives before a noun (like before ragione here), qualsiasi and qualunque mean the exact same thing and are completely interchangeable.

Is it mandatory to include the pronoun tu before abbia?

It isn't strictly mandatory, but it is highly recommended here. The present subjunctive forms for I, you, and he/she are all exactly the same (abbia). Adding tu prevents any confusion about who has the reason.

I learned that valere means 'to be worth'. Why is vale translated as 'applies' here?

While valere does mean 'to be worth', it is also commonly used to mean 'to be valid' or 'to be in effect'. When talking about rules, laws, or limits like questo limite assoluto, translating it as 'applies' or 'is valid' makes the most sense.