The catalog can be used for full fledged as well as for incremental data exchange. In both cases, all metadata have to be transmitted. In case of incremental exchange the unchanged physical files can be omitted.
In order to keep all references valid within the catalog the appropriate <ablock>s must be provided. Therefore, the [upd] attribute allows to give information about the contents:

NEW
The object is newly introduced in the system. Therefore the contents file must be in the catalog (in particular, an <file> element must occur within <ablock>).
REUSED
The object is introduced in the system at former time. Like NEW it is not in the reference catalog. Therefore the contents file must be in the catalog (in particular, an <file> element must occur within <ablock>).
UNUSED
The object is no longer used. The <ablock> is there in order to indicate, that the object can be removed in the configuration on the receiver's site. The <ablock> does not reference anything within the actual catalog.
This is used to support the update procedure on the receiver's site.
UNCHANGED
The object is unchanged. Therefore it must already be available on the receiver's site. Therefore it is possible to omit the file itself.
CHANGED
The object is changed. Therefore the contents file must be in the catalog (in particular, an <file> element must occur within <ablock>).
MOVED
The object is unchanged but in different Groups. Therefore it must already be available on the receiver's site. So it is possible to omit the file itself.
The object is changed and in different Groups. In this case it will be changed.