The number of the week in the year. First week of the year has number 1, last has number 52 or 53, depending on the year. Also depending on the method of calculation. Last time I checked, .NET didn't return week numbers according to the ISO standard so maybe the WP Calendar team didn't want to open that can of worms.
I believe it's just that week numbers are popular in some locales/markets/professions and pretty much ignored everywhere else.
As for .NET, I believe the ISO week numbering is equivalent to the CalendarWeekRule.FirstFourDayWeek. That's not very helpful, as the ISO rule is almost unanimously disregarded.