The BookNet Canada & Book Industry Study Group (BISG) Best Practices for Product Metadata is a comprehensive and useful resource for ONIX beginners or those fresh to ONIX 3.0, it contains:
A breakdown of ONIX blocks, highlighting what pieces of metadata are included within each
Use cases for each composite
Identification of mandatory tags/pieces of information
The complete document can be read below or downloaded at https://booknetcanada.atlassian.net/wiki/download/attachments/1378407/BISG_Best_Practices_for_Product_Metadata_6.1.15.pdf?api=v2.