>>96497420
the Raptor Imperialis is an honor marking used during the Great Crusade

the Aquila is the personal symbol of the Emperor and in widespread use by his personal guard the Legio Custodes as a symbol of his authority being vested in them, as well as used as an honour marking for certain figures and organisations (notably the Emperor's Children were granted the use of the Aquila and incorporated it into their own heraldry - Fulgrim wears it, dreadnoughts wear it, Praetors wear it, even Palatine Blades and others wear it)

as the Horus Heresy went on wearing the Aquila became a sign of allegiance for Loyalists and more commonly used, to the point that they were incorporating it directly onto new power armour during the Siege of the Imperial Palace at the end of the war

you do see some figures wearing a single-headed eagle (for example Rogal Dorn and Roboute Guilliman) which is likely an indicator of how important people signified their rank during the Great Crusade, or maybe used as an indicator of "junior" status of their own eagle heraldry compared to the "senior" two-headed or double eagle (it is after all a widespread heraldic device in many cultures across time, because eagles are cool)

there was some old fluff about the double-headed eagle representing the union of Mars and Terra but I think that's deprecated now

as far as your dudes go, if you don't want to use it, don't; if you do, see what kind of models already have it and use that as a rough guide to which of your dudes would have it