SOME states allow for reimbursement of sales taxes paid by the church. If you're not living in one of those states - ignore it.
More details can be found
here.
Ward and stakes outside these areas are not required to record taxes paid. For these wards and stakes there is no Tax category on the Expense screen.
Since you're asking the purpose, I'm guessing you're not seeing those fields in MLS.
I should note that there is no standard reimbursement form. Your ward may be using one that came from another area. (Earlier versions of MLS didn't hide the tax screen, so they might have thought they needed to enter it.)
Have you searched the
Help Center? Try doing a Google search and adding "site:churchofjesuschrist.org/help" to the search criteria.
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Profile to include your general location.