A fitting can also be a collection of other fittings. Per CSA B51-14 Table 1 Notes(2):

Category H can include (b) an assembly of components (including piping components), provided that the diameter of any component does not exceed 152 mm (6 in) and the total volume of the assembly does not exceed 42.5 L (1.5 ft3). Such an assembly is considered a single Category H fitting for the purposes of fitting registration;

This is like a miniature piping system. A recent requirement is that these assemblies of components must now be made from registered components. Sections of pipe are calculated. Custom components made by the manufacturer require calculations or burst tests or separate CRN registration. All other parts need CRNs.