The Russian defence ministry has been recruiting prisoners to fight in Ukraine, apparently taking over from the Wagner mercenary group which was the first to adopt the practice last year.
Such army units are commonly known as Storm-Z, the letter Z being one of the symbols of Vladimir Putin's so-called "special military operation" against Ukraine. It is also the first letter of the Russian word "zek", or "inmate".
The name Storm-Z is unofficial and can be applied to a range of Russian army units active in different parts of Ukraine.
Similarly to Wagner's prisoner units, Storm-Z detachments are reportedly often treated as an expendable force thrown into battle - with little consideration for the lives of their servicemen.
There are also indications that members of other army units can be sent to Storm-Z detachments as punishment for violations such as insubordination or drunkenness.
https://www.bbc.co.uk/news/world-europe-67175566