Fields are the elements within an article. A field represents the smallest database object that can be identified for replication. In tables, fields identify columns. In stored procedures, fields identify input parameters passed to the procedure when it is executed.
When you create a primary article, you can either publish all fields, or you can specify a subset of the fields for publication in that article. Likewise, when you create a replicate article, you can either subscribe to all published fields in the primary article, or you can specify a subset of the published fields to be subscribed to.
By specifying fields in both primary and replicate articles, you have two levels of selection to control the data replicated.