Thanks for being so kind and giving me a "wonderful wikitreer"!
Linietropperna is a part of the country of Norway's defence, I found an old book describing how it was organized, but I don't have much (or any) knowledge of the military in any country actually.
(Book "Kingdom of Norway and the Norwegian people")
"Norges Landvæbning har ifølge Lov af 12te Mai 1866 bestaaet af Linietropper med Reserve, Landeværn og Træn." (norwegian, and a kind of translation is "Norway's defence troops have according to the law of the 12th May 1866 been put together of Linietropper with the reserves, the land guard and the equipment staff".
I think you can translate Linietropper to Line regiment see wikipedia:
The "sessionssted" I don't find a good translation for, but as written Danish and Norwegian is quite similar, I found information on a Danish wikipedia page, that the session in a military meaning is the meeting / draft where the men to become soldiers meet up to be enlisted. (Another word seems to be conscription https://en.wikipedia.org/wiki/Conscription) so with this little discussion I guess that "sessionssted" is a conscription place.
So Ole might have been sent to a conscription place that was actually as far away as Gravdal. Then he was allotted to become a reserve, i.e. he did not have to train as a military (I guess).
Edit: I looked again for Gravdal. There seems to have been a military fort outside of Bergen, called Gravdal. See:
this is unsure, because it seems to have become a military training place more than 30 years after 1862. Perhaps a Norwegian wikitree member will know which Gravdal your ancestor is writing about. (It is for sure a name that can be used in more than one place, I found farms called Gravdal in Eid parish, Hordaland, see familysearch wiki:
(More Norwegian parishes with the place name Gravdal.)