Weil McLain EG Piping Question

WM installation has this notice:

"Steam supply must be on same end as controls. Return may be from either end."

See page 17 in their manual - https://www.weil-mclain.com/wp-content/uploads/EG-PEG-Series-6-Boiler-Manual-Web-Version.pdf

What do they mean by supply? The riser from the boiler or the supply takeoff from the header?

The Williamson manual, which looks like a photocopy of the WM manual, does not have this notice.