@Walter Hinteler,
Following the Franco-Prussian War in 1870/71, conscription was introduced in all German states.
Effective military service began then upon a man’s 20th birthday. If the service was in the infantry it was for two years and for three in the cavalry and the artillery.
The soldier then passed into the Reserve for either four or five years according to the number of active years he had just completed.
Additionally, the German reservist was required to undergo a fortnights training every year.
For the next eleven years the soldier served in the "Landwehr" from where he passed (back) into the "Landsturm" for a further seven years. At the ripe old age of 45 he was no longer eligible for military service.
It was only intended to call up the Landwehr and Landsturm in time of actual war.
Each year there were more twenty year olds than places in the army.
In 1900 therefore the idea of an "Ersatz-Reserve" came into being. Ersatz means supplement and the Regiments contained those men who for one reason or other had not been called up. They were required to serve for twelve years with the possibility of three training sessions each year (which rarely happened).
An "Einjährig-Freiwilliger" (one-year volunteer) was a voluntary conscript with secondary school education ("Obersekunda"). The "one-year volunteer service" was first introduced 1814 in Prussia, and was taken over by the German Empire from 1871 until 1918, and was also implemented in the Austro-Hungarian Heer from 1868 until 1918.
In nearly all German local/regional museums, you can see items (steins and other memorabilities) of the day, when the reservists left the active duty - they didn't like or like the conscripted period like anyone else generations later.
Since patriotism, after the 70/71 war and the unification, was 'in', German families liked to show proudly their sons ... sometimes even 'upgraded' through hand-colouring - like this one of a great-uncle
But even, if you didn't like the military, as a one-year volunteer you could get promoted to cadet-officer and prove this with a sable, like (paternal) grandfather