For example if the corporate tax rate is 26%, then Tax Expense divided by Profit Before Tax should be around that range. Differences usually arise because of either:
1. Timing issues (eg. expenses being tax deductible in a different accounting period)
2. Permanent differences (eg. expenses not being deductible under law, or certain income not taxable under law)

If a company’s tax expense is substantially less than the corporate tax rate, then it pays to analyse the numbers in more detail.

It is a measure of the company’s earnings margin before the impact of tax and financing.