user30017: I understand that BIP91 needs 269/336 signaled blocks.
My question is is it a rolling window of any 336 blocks or are there activation periods of 336 blocks that start and end at predetermined block numbers?

user48884: It looks like it is actually periods of 336 blocks instead of a rolling window. It is like what BIP 9 specifies, but uses 336 blocks as the window and 269 blocks as the threshold. The windows begin when a block's height is a multiple of 336.

30017: <p>I understand that BIP91 needs 269/336 signaled blocks.<br>
My question is is it a rolling window of any 336 blocks or are there activation periods of 336 blocks that start and end at predetermined block numbers?</p>
 48884: <p>It looks like it is actually periods of 336 blocks instead of a rolling window. It is like what BIP 9 specifies, but uses 336 blocks as the window and 269 blocks as the threshold. The windows begin when a block's height is a multiple of 336.</p>
