39.—This Chapter specifies the effect an order to pay or, as appropriate, an assessment has for various purposes (including the purposes of certain provisions of the Act of 1961); a different effect is specified by reference to the different circumstances in which an order to pay or an assessment falls to be considered and, in particular, circumstances—

(a) in which there is a default in payment of the amount or amounts specified in an order to pay, and

(b) in which a respondent who has paid the amount or amounts specified in an order to pay seeks to make a claim for contribution against one or more concurrent wrongdoers in respect of the relevant claim concerned.