TTL is the time to live for your own queries.

SOFT_MAX_TTL is practically the maximum value for hops + ttl of any message that is passed through your client. If hops+ttl > SOFT_MAX_TTL the ttl will be reduced.

setting MAX_TTL (unless you set it lower than soft_max_ttl) doesn't have any effect anymore in the network as it is now. It was used to set the maximum value for hops+ttl, until the message would be dropped without normalizing the ttl.