29 U.S.C. § 1381

1. (a)If an employer withdraws from a multiemployer plan in a complete withdrawal or a partial withdrawal, then the employer is liable to the plan in the amount determined under this part to be the withdrawal liability.
2. (b)For purposes of subsection (a)—
   1. (1)The withdrawal liability of an employer to a plan is the amount determined under section 1391 of this title to be the allocable amount of unfunded vested benefits, adjusted—
      1. (A)first, by any de minimis reduction applicable under section 1389 of this title,
      2. (B)next, in the case of a partial withdrawal, in accordance with section 1386 of this title,
      3. (C)then, to the extent necessary to reflect the limitation on annual payments under section 1399(c)(1)(B) of this title, and
      4. (D)finally, in accordance with section 1405 of this title.
   2. (2)The term “complete withdrawal” means a complete withdrawal described in section 1383 of this title.
   3. (3)The term “partial withdrawal” means a partial withdrawal described in section 1385 of this title.