Answer
This depends entirely on which platform generation you're on. On the older Apps10 release line, the delivered source code is owned and maintained by IFS in a source repository you don't have write access to — you cannot modify it directly, doing so would typically breach the support agreement, and any change would likely be overwritten by the next code delivery if it touches the same objects; instead, standard customization mechanisms (custom objects, custom events, custom fields) are the supported route, and it's worth involving your IFS contacts on what's achievable that way. On IFS Cloud, this changes fundamentally: application code lives in the Customer Buildplace and UsePlace, which the customer owns outright, opening up the ability to make direct code-level changes yourself — a capability that simply doesn't exist on the older, IFS-owned Apps10 codebase.