(a) Principal divisions. Internal revenue taxes fall generally into the following principal divisions:

(1) Taxes collected by assessment.

(2) Taxes collected by means of revenue stamps.

(b) Assessed taxes. Taxes collected principally by assessment fall into the following two main classes:

(1) Taxes within the jurisdiction of the U.S. Tax Court. These include:

(i) Income and profits taxes imposed by Chapters 1 and 2 of the 1939 Code and taxes imposed by subtitle A of the 1954 Code, relating to income taxes.

(ii) Estate taxes imposed by Chapter 3 of the 1939 Code and Chapter 11 of the 1954 Code.

(iii) Gift tax imposed by Chapter 4 of the 1939 Code and Chapter 12 of the 1954 Code.

(iv) The tax on generation-skipping transfers imposed by Chapter 13 of the 1954 Code.

(v) Taxes imposed by Chapters 41 through 44 of the 1954 Code.

(2) Taxes not within the jurisdiction of the U.S. Tax Court. Taxes not imposed by Chapter 1, 2, 3, or 4 of the 1939 Code or Subtitle A or Chapter 11 or 12 of the 1954 Code are within this class, such as:

(i) Employment taxes.

(ii) Miscellaneous excise taxes collected by return.

(3) The difference between these two main classes is that only taxes described in subparagraph (1) of this paragraph, i.e., those within the jurisdiction of the Tax Court, may be contested before an independent tribunal prior to payment. Taxes of both classes may be contested by first making payment, filing claim for refund, and then bringing suit to recover if the claim is disallowed or no decision is rendered thereon within six months.