You can answer this question on two levels, the explicit and the implicit. On the explicit level, Bluntschli gives them explicit instructions to tell the men that they must leave at once, and make sure the orders are delivered by the deadline. His words indicate that they should take a...
You can answer this question on two levels, the explicit and the implicit. On the explicit level, Bluntschli gives them explicit instructions to tell the men that they must leave at once, and make sure the orders are delivered by the deadline. His words indicate that they should take a threatening, authoritarian tone -- make the men know that any deviance from instructions will result in corporal punishment.
BLUNTSCHLI. You had better both see the fellows that are to take these. [To Sergius] Pack them off at once; and shew them that I've marked on the orders the time they should hand them in by. Tell them that if they stop to drink or tell stories--if they're five minutes late, they'll have the skin taken off their backs.
After Sergius leaves, Bluntschli also asks Petkoff to go after him and make sure the instructions get carried out.
On the implicit level, Bluntschli's words imply his criticism of the way things usually get done by these Bulgarian officers.
Bluntschli is a seasoned, pragmatic, efficient, and professional soldier. He knows from experience that messengers are liable to dally without threat of strict discipline -- perhaps especially after the declaration of peace and the demobilization of many troops. Bluntschli has seen, from observation, that Sergius is full of romantic, unrealistic notions about military matters, and so he thinks Sergius needs this practical advice. Left to his own devices, Sergius is liable to address his men as if they will discharge their orders purely out of honor and devotion to him as their noble superior.
This suspicion is corroborated by Sergius's immediate reply to Bluntschli's instructions:
SERGIUS [rising indignantly] I'll say so. And if one of them is man enough to spit in my face for insulting him, I'll buy his discharge and give him a pension. [He strides out, his humanity deeply outraged].
To Sergius, Bluntschli's approach is insulting, because it presupposes that the men aren't intrinsically motivated by honor and heroic ideals. Petkoff -- favorably impressed by Bluntschli's competence and less obsessed than Sergius with heroics -- doesn't show indignation. He also seems more open to the implicit message. Petkoff has already acknowledged that he is less adept than Bluntschli in the handling of practical military matters.