By convention, gb elements should appear at the start of the first page in the gathering. The global n attribute indicates the number or other value used to identify this gathering in a collation.
The type attribute may be used to further characterize the gathering in any respect.
Example
<gb n="iii"/> <pb n="2r"/> <!-- material from page 2 recto of gathering iii here --> <pb n="2v"/> <!-- material from page 2 verso of gathering iii here -->