Un-stressed reinforcement is added to provide an ultimate moment capacity greater than the cracking moment.

The cracking is assumed to be top (hogging moment) or bottom (sagging moment) based on the "Min. Reinforcement Pattern" selected in the design strip segment or design section.

Only sections within 1/6 of the span length from supports or 1/6 of the span length from midspan are checked, as these are considered as the likely locations of first cracking of concrete.

See Cracking Moment Used in Design Calculations for a theoretical discussion of the cracking moment (note that the 1.2 factor is not used in BS 8110)

The cracking stress is $0.6\sqrt{f_{\mathrm{cu}}}$. For cross sections with multiple concrete mixes, the maximum f_cu for the cross section is used.