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The format is the same for (almost) all visas in the Schengen area but those fields are most relevant for short-stay type C visas. Thus a short-stay visa could be restricted to one or two entries or have a limitation on the duration of stay. In that case, “Number of entries” would read “1” or “2” and “Duration of stay” could for example be “10 days”.
“Number of entries: MULT” means you can leave and reenter the Schengen area as many times as you wish, which is (almost?) always the case for long-stay visas but the field is there anyway.
For visas longer than six months (either type C or D), the maximum duration of stay is always 90 days in any 180-day period. For a type C visa, it means you can stay up to 90 days in the whole Schengen area in any 180-day period. For a type D visa like yours, it means you can stay up to 90 days in any 180-day period in other Schengen member states than Germany (where you can always stay for the whole duration of your visa).
Since such visas allow the holder to stay in one country for longer than 90 days (either at once for a type D visa or during multiple stays for a long type C visa) while still having some restrictions, the field is sometimes marked “XX”. In your case, XX means you can stay up to 133 days in Germany between the 25th of August and the 5th of January and up to 90 days elsewhere in the Schengen area between those dates, which is why writing either “90 days” or “133 days” would be even more confusing.