The rail item a contribution joins when it replaces sectionKey.
Replacing something means standing where it stood. A panel put in place of a field
section belongs in the Details tab that section was part of; one put in place of the
whole Details group belongs in that group; one put in place of a leftover audit section
belongs in More. Null means the contribution keeps a rail item of its own: it replaces
nothing, the section is not on the form, or it takes over a related grid — that last one
already inherits the grid's rail item through the relationship claim.
Without this a replacing panel is lifted to a first-class rail item beside the tab it
emptied, so the form loses a section and gains a tab.
The rail item a contribution joins when it replaces
sectionKey.Replacing something means standing where it stood. A panel put in place of a field section belongs in the Details tab that section was part of; one put in place of the whole Details group belongs in that group; one put in place of a leftover audit section belongs in More. Null means the contribution keeps a rail item of its own: it replaces nothing, the section is not on the form, or it takes over a related grid — that last one already inherits the grid's rail item through the relationship claim.
Without this a replacing panel is lifted to a first-class rail item beside the tab it emptied, so the form loses a section and gains a tab.