For purposes of RCW 50.24.210, an employer is considered to have quit business or disposed of its business or stock of goods if it disposes of substantially all of its operating assets. An employer is also considered to have quit business or disposed of its business or stock of goods if it transfers operating assets and retains only assets that do not have substantial net value or that are lower in value than total unemployment taxes, penalties, and interest owed. If an employer quits business or disposes of its business or stock of goods and has more than one successor, all successors are jointly and severally liable for any unemployment taxes due unless the employer and all successors have notified the department in writing and the department has approved apportioning any unemployment tax liability between the successors.