As part of the runtime fields project, we're need a unique way to identify runtime fields in the Kibana UI. These are custom created fields - created in the UI - that will appear in a variety of places (field lists, index pattern management, etc.). As they behave and perform differently, it's necessary to denote fields of this type.

@MichaelMarcialis proposed the icon, below, for Lens and I've since suggested it for the Index Patterns UI, as well. Both efforts are in progress and will be using a temporary/similar icon for the time being.

Something akin to icons in the indexX... family feels appropriate.