task: megablast versus blastn
Task types define optimal settings (word-size, gap values) for comparing inner- or inter-species sequences.
-task megablast (default): for comparison within the same species (fast)
-task blastn: for comparison of sequences from different species (slow, but finds more hits)
"The “megablast” task is optimized for intraspecies comparison as it uses a large word-size of 28bp, whereas “blastn” is better suited for interspecies comparisons with a shorter word-size of 11bp." [ncbi]