Except as provided in paragraph (2) of this subsection, this section applies only to employees in the skilled service or the professional service. This section does not apply to an employee under a special appointment described in § 6-405 of this article. Except as provided in paragraph (3) of this subsection, an employee in the skilled service or the professional service may appeal a disciplinary action taken while the employee is on probation only on the basis that the action was illegal or unconstitutional. The employee has the burden of proof in an appeal under this subsection. The limitations in paragraphs (1) and (2) of this subsection do not apply to an employee in the skilled service or the professional service who is on probation following a promotion or reinstatement. An employee or an employee’s representative may file with the head of the principal unit a written appeal of a disciplinary action that states, to the extent possible, the issues of fact and law that the employee believes would warrant rescinding the disciplinary action. An appeal under this subtitle must be filed within 15 days after the employee receives notice of the appointing authority’s action. The head of the principal unit may confer with the employee before making a decision. The head of the principal unit may: uphold the disciplinary action; or rescind or modify the disciplinary action taken and restore to the employee any lost time, compensation, status, or benefits. Within 15 days after receiving an appeal, the head of the principal unit shall issue to the employee a written decision that addresses each point raised in the appeal. Within 15 days after issuance of a decision to rescind a disciplinary action, the disciplinary action shall be expunged from the employee’s personnel records.