I am reading the AN4776, General-purpose timer cookbook. At the 9th page, it mentioned the repetition counter is null. What does a register is null really mean？？

[waclawek.jan]

That its content is 0.

JW

[Uwe Bonnes]

Repetion count null means that at rollover an update event is generated. If it is non null, no update event is generated, but the RCR is decremented