It is a non-standard post attribute that could come from the ‘post custom fields feature’ or other plugins that extend the post’s meta-data. The most known such plugins are custom fields providers, e.g. Meta Box – WordPress Custom Fields Framework.
As a result, any custom fields have an ID and vale. So, these fields should be added as separate body attribute with name equals to that ID.
In-place edit mode
In in-place edit mode, these attributes are set in the post’s meta-data editors with the same ID.
Note: Common practice is to prefix ID with ‘_’ to hide it from ‘post custom fields feature’.