Extend Tool Options

The following options are available in the Options panel:

Maximum distance The maximum distance between parts. The tool will search for adjacent faces again if you change this value.
Trim surfaces Controls whether or not surfaces may be trimmed in addition to extended.
Partial intersections Controls whether or not faces that partially intersect are detected.
Same body Allows a surface to be trimmed or extended by a face or edge on the same body.
Extend to curves Finds surfaces to extend to curves when the curve is in the same plane as the surface.
Merge after extend or trim Merges bodies, if possible, when you trim or extend an edge on one surface body up to a face or edge on another body.