In the BU system, node operators can choose the blocksize limit they vote on by modifying their client via a GUI menu and miners choose to increase or decrease the blocksize according to what nodes are voting on. This would, in theory, create a system where Bitcoin scales according to the space needed to accommodate the transactions in the network which is prone to keep growing like Bitcoin’s popularity and user base.
This system allows users to choose the maximum block size they want to accept as valid. However, there is a user-configurable failsafe mechanism that allows users to accept a block larger than the limit they have previously set if the number of blocks above this limit reaches a certain number. This system ensures that no users are accidentally left behind.