For template_type and template_length, this is all I can really find on them

 -template_type <String, `coding', `coding_and_optimal', `optimal'>
   Discontiguous MegaBLAST template type
 -template_length <Integer, Permissible values: '16' '18' '21' >
   Discontiguous MegaBLAST template length

I don't really know what they are though and I don't know what differences setting optimal or coding will do. I also don't know the benefit of selecting 16 18 or 21 as length

This is all I can find on the best hit options

-best_hit_overhang <Real, (>0 and <0.5)>
   Best Hit algorithm overhang value (recommended value: 0.1)
 -best_hit_score_edge <Real, (>0 and <0.5)>
   Best Hit algorithm score edge value (recommended value: 0.1)