Tax Debts and other priority claims:
a.    Payment in full required under ch. 13.  Certain tax debts are priority claims under 507(a)(8), and so have to be paid in full under a ch. 13 plan under 1322(a)(2).
b.    Tax lien:  If IRS is fully secured, full payment over time must reflect present value of debt as of the effective date of the plan under 1325(a)(5).
c.    IRS claim based on tax fraud doesn’t have priority under 507(a) and so doesn’t have to be paid in full under 1322(a).
d.    Any tax claim for that doesn’t have priority under 507 will be treated as a general unsecured claim.
e.    Ch. 13 stops the running of post-pet interest on this debt.  Under ch. 7, this debt is non-dischargeable, so you just keep paying it according to the IRS’s terms.  Under Ch. 13, the plan needs to make payment in full, but just have to pay in full based on what debt is at the time.
f.    Ch. 13 payments are involuntary, so IRS gets to allocate them to whatever debts it wants to.  Ex.  taxpayer has dischargeable and non-dischargeable tax debt.  IRS can allocate payments to non-dischargeable debt first,  and leave dischareable debts still to be paid.