Ability to select any custom field for grouping without adding it first through "Fields" tab of "Custom Timesheet"

Dear team,

While viewing the timesheet, we see a certain list of fields as "available" to select.

Could you please suggest what rule is followed to show a field here? (attached screenshot)

We can select a field every time we select timesheet and add from the "Fields" tab but to avoid doing so every time, can we have a way to always show a custom field in this list?