[granzer]

I read the definition for the BoundayField as "BoundaryField This is a GeometricBoundaryField, in which a Field is defined for the faces
of each patch and a Field is defined for the patches of the boundary. This is then a field of fields, stored within an object of the FieldField<Type> class. A reference to the fvBoundaryMesh is also stored [**].". This part, 'a GeometricBoundaryField, in which a Field is defined for the faces
of each patch', made sense. But what does 'a Field is defined for the patches of the boundary' mean?. For a given Field how can two different values be assigned at a boundary patch(as I understand one field assigned for faces of each patch and one for the 'patches of the boundary')?

[granzer]

Ok as I have come to understand it now, the closed boundary of a computational domain is broken down into patches, and each of the patches is further broken down into individual faces.

"BoundaryField This is a GeometricBoundaryField, in which a Field is defined for the faces of each patch..."

means exactly what it says...that is a field value, like for example temperature value(if a 'temperature field' is being defined over the patch), is given for each of the faces in a patch. So all the temperature values at the boundary face centers make up the boundary 'field'/list.

"...and a Field is defined for the patches of the boundary."

This means that since the boundary of the computational domain is broken down into patches, we can view the boundary as a field/list of patches (these patches are further broken down into faces which are talked about in the previous para).

So GeomethricBoundaryField is a field of fields, fist level down is the list/field of patches.. the second level down is the list/field of field-values at individual faces. (like a 2d array)