as ron says, the zulu amabutho (regiments) were military regiments, their food was supplied, and equipment was 'government issue' and not personal and shields and weapons were considered the property of the king.
the members of the ibutho (regiment), all male of the same age group, lived, trained, worked together in fortified military encampments, did not marry until retired and permitted by the king, and were very much like roman legionaries*. each ibutho, like the romans, had their own insignia, the color and striping of their shields also designated the ibutho.
'impi' just means an armed zulu band, maybe 10 maybe 100, maybe 10,000 and consisting of a small part of a regimental ibutho, or maybe 3 or 4 amabutho acting in concert.
*- the rank and file roman soldiers were legionaries (generic milites - soldiers, or munifex - the equivalent of a private), 80 legionnaires were commanded by a centurion, there were about 60 centurii in a legion, see
Roman Legion which expalins the rank structure in detail.