You can specify SDFs either statically (you specify the actual name of the file) or dynamically (you use a FILLER field as the source of the file name). In either case, when the EOF of an SDF is reached, the file is closed and further attempts at reading data from that particular file produce results equivalent to reading data from an empty field.
In a dynamic secondary file specification, this behavior is slightly different. Whenever the specification changes to reference a new file, the old file is closed, and the data is read from the beginning of the newly referenced file.
The dynamic switching of the data source files has a resetting effect. For example, when SQL*Loader switches from the current file to a previously opened file, the previously opened file is reopened, and the data is read from the beginning of the file.
You should not specify the same SDF as the source of two different fields. If you do, then the two fields will typically read the data independently.