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.