The figure for a highly compensated employee is your total compensation from your company.

It wouldn't be your taxable income, as you could elect to take a portion of your compensation and put it into your 401k to lower your taxable income, or have a heavily leveraged very expensive home and a very large home interest deduction to lower your taxable income. Neither of these things affects your compensation amount, though, and that is what the term compensation is referring to in the term Highly Compensated Employee.

There are various legal definitions "highly compensated" for benefits plan purposes. The definition varies based on which benefit plan you are referring to and subsequently which non-discrimination test you may be trying to pass. Not all use $110,000 as an on/off switch to determine highly compensated.

"Highly Compensated" refers to gross pay no matter which definition you use and no matter what non-discrimination test you are trying to pass.

From a tax rate perpective it relates to taxable income.