The parameter ed defines the average edge length of tetrahedral zones.
The alternate keyword may be used for blocks that do not zone properly using the normal schemes (see below). In this case, the block is meshed independent of adjacent blocks, and gridpoint matching is performed post-zoning. In general, this scheme leads to more-poorly shaped zones and should be used as a last resort.
Retrying zone generation with either a longer or shorter length can be done safely. Retrying zone generation with a different edge length often produces good results. For flat or narrow blocks, sometimes a shorter length is better. For large blocks, the edge length may have to be increased.