20 C.F.R. § 345.207
If the Board determines that an employer has permanently ceased to pay compensation with respect to which contributions are payable under this part, the Board will, on the date of such determination, transfer the employer's net cumulative contribution balance as a subtraction from, and the cumulative benefit balance as an addition to, the system unallocated charge balance and will cancel all other accumulations of the employer. The Board's determination that an employer is defunct will be based on evidence indicating that the employer has ceased all operations as an employer and has terminated its status as an employer. In making its determination, the Board will consider evidence as described in part 202 of this chapter with respect to termination of employer status under the Railroad Retirement Act. Mere failure of an employer to pay contributions due under this part does not indicate that such employer is defunct.
20 C.F.R. § 345.207