You'll want to study the Migration Guide (and more):
http://helpcenter.graphisoft.com/guides/archicad-17-int-reference-guide/migration-guide-for-archicad...
[Link above updated 7/15/13]
As it says there:
The names of the new Building Materials are generated from the original Cut Fill’s name and will be extended with a unique nine-digit identifier
The more inconsistent the usage of fills, priorities, pens, etc in the original file, the more numeric-suffixed cut fill named BM's will be generated.
In the long run, Building Materials will save us all tons of work, but migration will take time. As Rick Thompson says in another thread - and is mentioned in the migration article - it can pay to clean things up some in the original 16 file first. Then, the new delete/merge capability with attributes helps clean up things further.
See also:
http://helpcenter.graphisoft.com/guides/archicad-17-int-reference-guide/configuration/attributes/bui...
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