There are a multitude of options available in how selectivity for fleets and indices is parameterized in the model. This vignette is designed to be a step-by-step guide for setup of selectivity.
Additional features for modeling the catchability coefficient for indices are considered as well.
This article is designed to provide step-by-step information on possible model configurations, but of course it is up to the user to decide whether those configurations are appropriate for the operating model conditioning.
It will be best to first think about fleet selectivity followed by index selectivity and catchability.