Usually you should specify "access" attribute.
There are mapping errors when someone refactors code and converts field to/from property and forgets to change attribute value.

There is some automatisation:
1. If you specify "access" explicit - it will be used
2. If "access" is not specified - NHMA will determine if it is "field" or "property"

It works for all attribute types except ComponentAttribute, there you should specify "access" manually.

Pull request: https://github.com/nhibernate/NHibernate.Mapping.Attributes/pull/2