With area engraving the laser moves back and forward, line by line, similar to the way in which an inkjet printer applies ink, but instead of ink being applied, material is removed by the laser.
Line engraving, often referred to as “scoring", is like shallow cut. The laser traces along the paths in your design file, lightly marking the surface. The width of line engraving is approx. 0.2mm, the width of the laser beam.
When you choose to combine area & line engraving you'll get both laser processes on the same artwork. First the laser will do the area engraving and then it'll line engrave the profile. Typically this makes the edges crisper and helps the engraving to pop.
Here are some examples of engraving on amber bamboo vs black acrylic.
You can see that using both area and line engraving (aka combined engraving) shows up best on bamboo:
On black acrylic however, just area engraving alone looks best (as long as areas are above 0.3mm wide - thinner sections can be missed and would be better as line engraving):