It's been a long time, but I think the examples Heinlein gave were terraforming on Venus, and testing experimental equipment. Somewhere Pluto got into there, too.
I don't know if these positions carried military rank with them, or not, but I suppose they did. It's not soldiering, though.
One example was counting hairs on fuzzy caterpillars.
That was given as an example of how the federal government could find find something that any applicant could do, even if they were a blind quadriplegic, should they wish to earn citizenship.
I can’t see that as a particularly hazardous duty.