Posteno is kind of a profession, while laboro is more like a task? (Not exactly what I mean, but ...)
Posteno is like a role to which you get appointed ("CEO", "senior administrator", "chief bottlewasher"), and laboro is more like "work" (the work you produce) - I suppose "task" would also work.
Dankon. Mi nun komprenas. ;)
Li volas esti instruisto esperanta... ;-)
Cxu vi usas "career" anstatau "job"?