SIL norms define different rules that must be applied to code (i.e. : cyclomatic complexity, etc.)
These rules are defined by "NA - Non Applicable", "R - Recommended" and "HR - Higly Recommended".
I understand this appreciation is up to the people that define the software.
How constrained am I to follow the "R" rules from the "HR" rules ? Are the first one recommended and the second mandatory ?
It is not clearly defined in the standards, so it is subject to interpretation by the certification authority. There are also variations in interpretation between standards (e.g. IEC 61508 vs EN 50128)
Most auditors would consider HR mandatory without some documented justification.
R is generally considered optional, but you generally need to select some of those options. (As opposed to optional meaning "can be ignored")