By 600, the establishment of the first Anglo-Saxon 'emporia' was in prospect. There seem to have been over thirty of such units, many of which were certainly controlled by kings, in the parts of Britain which the Anglo-Saxons controlled. Bede's use of the term "imperium" has been seen as significant in defining the status and powers of the bretwaldas, in fact it is a word Bede used regularly as an alternative to "regnum"; scholars believe this just meant the collection of tribute. Oswiu's extension of overlordship over the Picts and Scots is expressed in terms of making them tributary. Military overlordship could bring great short-term success and wealth, but the system had its disadvantages. Many of the overlords enjoyed their powers for a relatively short period. Foundations had to be carefully laid to turn a tribute-paying under-kingdom into a permanent acquisition, such as Bernician absorption of Deira. The smaller kingdoms did not disappear without trace once they were incorporated into larger polities; on the contrary their territorial integrity was preserved when they became ealdormanries or, depending on size, parts of ealdormanries within their new kingdoms. An example of this tendency for later boundaries to preserve earlier arrangements is Sussex; the county boundary is essentially the same as that of the West Saxon shire and the Anglo-Saxon kingdom. The Witan, also called Witenagemot, was the council of kings; its essential duty was to advise the king on all matters on which he chose to ask its opinion. It attested his grants of land to churches or laymen, consented to his issue of new laws or new statements of ancient custom, and helped him deal with rebels and persons suspected of disaffection.