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They've run out of troops on the front line, and cannot replenish them quickly enough from volunteers (or prisoners, etc).
All those on the reservists are basically being called up in a prioritised order, initially 300k are being called up. Total reservists is 2M.
The decree to do that is signed (attempting to leave Russia or not respond when called makes you a deserter).
"Partial" in that this is not a general conscription at this time, and only a portion of the total reserves.
"partial" mobilisation.