Tag Properties
At 0%, only the Min. value defined in the "Parameters" tab is applied. At 100%, the entire value range between Min. and Max. is run through; for intermediate values, the Maximum value is reduced accordingly (higher values than 100% are of course also possible).
Fields can basically operate 3 channels, which you can select here. The most commonly used is undoubtedly Value, as this generates values between 0 and 1, which generates corresponding parameter values between the Min. and Max. values.
Load this scene once, in which 3 Field Driver tags work in the 3 different evaluation modes.
Value
Generates values between 0 and 1, which are used to generate parameter values between the limits (min./max.).
Color
This can be used to assign a field color (note the Color Mapping tab in the corresponding Field object, where color gradients can be loaded, for example).
Direction
This option is used to assign direction vectors and therefore only vectors are controlled. Most fields can specify directions as described at 
Field Force.
A Field Driver Tag only ever provides one value internally, which is used to define parameter values. The tag therefore needs a position where the field value can be determined. You define this position here.
This means that the Field Driver tag cannot work at point level or with elements that do not have a position.
Object Location
The object origin is used.
Geometry Center/Oriented Bounding Box
With Geometry Center, all object points are used and averaged, i.e. the position tends towards regions of high point density. This mode can be used for deformed objects, for example, where the object origin may deviate greatly from the deformation position.
Oriented Bounding Box works in the same way, except that the original axis alignment of the object is also retained, while Geometry Center always uses the world coordinate alignment.
When using e.g., the Cloth tag (also Soft Body and Balloon) or Rope tag, you should use one of these two modes.
Overwrite Sampling Location
You can use this option to specify any object position that is used to define the field value. To do this, drag a freely positionable null object into the Overwrite Sampling Position field, for example.