The distinguishing feature of a lap steel ukulele is that the strings are raised at both the nut and bridge ends of the fingerboard.
Round-necked resonator ukuleles set up for steel playing fall into this category.
Ukuleles which are made exclusively for slide playing may have a "square" profile neck.
Alternatively, the lap steel ukulele player can play the melody or another single part.
The lap steel ukulele appears to be a very recent development, with only a handful of professional musicians and large-scale manufacturers addressing the style and instrument.