the plugin, starting from upcoming QGIS release 3.16, will be replaced by newly available overlay functions: https://github.com/qgis/QGIS/pull/38405
the QGIS plugin provide a 'Reference' group under field calculator with function for analytical or spatial reference to featurse in other layers.
Uninstalling plugin removes funtions from field calculator
dbvalue(targetLayer,targetField,keyField,conditionValue)
Retrieve first targetField value from targetLayer when keyField is equal to conditionValue
dbvaluebyid('targetLayer','targetField',featureID)
Retrieve the targetField value from targetLayer using internal feature ID
dbquery(targetLayer,targetField,whereClause)
Retrieve first targetField value from targetLayer when whereClause is true
dbsql(connectionName,sqlQuery)
Retrieve results from SQL query
WKTcentroid('WKTgeometry')
Return the center of mass of the given geometry as WKT point geometry
WKTpointonsurface('WKTgeometry')
Return the point within the given geometry
WKTlenght('WKTgeometry')
Return the length of the given WKT geometry
WKTarea('WKTgeometry')
Return the area of the given WKT geometry
geomRedef('WKTgeometry')
redefine the current feature geometry with a new WKT geometry (experimental!)
geomnearest(targetLayer,targetField)
Retrieve target field value from the nearest target feature in target layer
geomdistance('targetLayer','targetField',distanceCheck)
Retrieve target field value from target feature in target layer if target feature is in distance
geomwithin(targetLayer,targetField)
Retrieve target field value when source feature is within target feature in target layer
geomtouches(targetLayer,targetField)
Retrieve target field value when source feature touches target feature in target layer
geomintersects(targetLayer,targetField)
Retrieve target field value when source feature intersects target feature in target layer
geomcontains(targetLayer,targetField)
Retrieve target field value when source feature contains target feature in target layer
geomcontains(targetLayer,targetField)
Retrieve target field value when source feature is disjoint from target feature in target layer
geomequals(targetLayer,targetField)
Retrieve target field value when source feature is equal to target feature in target layer
geomtouches(targetLayer,targetField)
Retrieve target field value when source feature touches target feature in target layer
geomoverlaps(targetLayer,targetField)
Retrieve target field value when source feature overlaps target feature in target layer
geomcrosses(targetLayer,targetField)
Retrieve target field value when source feature crosses target feature in target layer