The onus of proving a tax exemption where the guidelines aren’t clear would be on the assessee, and he cannot claim the benefit of any such ambiguity in provisions, a constitution bench of the Supreme Court said. Experts said the ruling would completely alter precedents as also the principle of law where the assessee was given the benefit of doubt. This will have significant ramifications for taxpayers and influence decision on past as well as future litigation over tax disputes, they said.