C24 high strength graded timber joists, often specified by architects and used in building projects where premium strength is required. Length of sawn and treated timber, regularised to a consistent size, with eased edges for easier handling, and strength graded to C24.
C16 refers to the softwood strength class. A strength class is simply a group of species/strength grade combinations which have similar strength/mechanical properties. There are a number of C strength classes (C14-C50) but C16 and C24 are the two most commonly used.