Have a read of the following link, it will give you a better understanding of what it is and what it is not.

https://en.wikipedia.org/wiki/Bitcoin_Unlimited

To quote:

Bitcoin Unlimited (BU) is a full node software client for the bitcoin network. Compared to the Bitcoin Core client hard-coding the block size limit to 1 megabyte, from which it is forked, Bitcoin Unlimited does not hard-code the limit, allowing the users to signal which block size limit they prefer, find the limit having a majority consensus and set their block size limit to that value.[1]

The release of Bitcoin Unlimited follows the release of Bitcoin XT and Bitcoin Classic, alternative proposals which aimed to increase bitcoin's transaction capacity of around 2.5-3 transactions per second by increasing the hard-coded block size limit.