"Johnson, stand here and guard the road against the approaching enemy" is a lawful order, but it may not be reasonable under many different interpretations.
This is pretty basic military law that a lawful order has a valid military purpose and is a clear and specific (and its generally documented in writing although verbal lawful orders do exist).
Reasonable is in the sense of proportionate such as "reasonable force". Would a reasonable person do X Y or Z to reach a lawful goal?
If you're guarding a nuclear bunker and there are signs everywhere about deadly force authorized and someone tries to break in, its a lawful order to shoot them although if they're a pizza deliveryman it may not be a reasonable order; although lets be realistic pizza deliverymen don't normally break into nuclear bunkers, so its perfectly reasonable to shoot a deliveryman-impersonator commando.
A very off the cuff and unfair comparison is the people who decide acceptability of lawful orders are skilled knowledgeable bureaucrat lawyer types implementing the details of written laws and regs and higher level orders, whereas the people who decide reasonableness of orders are usually on the knowledge level of jury members. Or lawful orders are in the arena of goals, whereas reasonable orders are in the arena of how to do it.
