This is a simple one but many people get the syntax wrong when they try to export individual document libraries / lists.
To backup a document library called ‘Library1’ in ‘Subsite’.
export-spweb <http://sharepoint site/site collection/subsite/ -itemurl “/site collection/subsite/library1” -path c:\sharepoint\library1.cmp
To backup a document library called ‘Library 2’ in ‘Site Collection’.
export-spweb <http://sharepoint site/site collection/ -itemurl “/site collection/library2” -path c:\sharepoint\library2.cmp