It’s a non-standard post attributes that could come from ‘post custom fields feature’ or other plugins that extend post meta-data. The most known such plugins are custom fields providers.
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, this attributes are set in post’s meta-data editors with the same ID.
Note: ‘post custom fields’ currently is not supported. If it is needed, please, write us.
In direct\batch mode, this attributes are stored in post’s meta-data with the same ID.
Note: Common practice is to prefix ID with ‘_’ to hide it from ‘post custom fields feature’.