The M+S marking stands for 'Mud + Snow'. The manufacturer can choose to apply this optional marking to the sidewall of a tyre if they believe that it's capable of driving on muddy and/or snow-covered roads.
Only the 3PMSF label, represented by an image of a three-peaked mountain surrounding a snowflake, is an official winter tyre certification.
Therefore, all '3PMSF' tyres have 'M+S' markings by default, but the opposite is not true. If you want official winter tyres, you must choose a tyre with the'3PMSF' label.