The UNITE Fund is a new type of product that, once purchased, grants additional one-time rewards with every Garage Level upgrade.
Once purchased, you own the UNITE Fund permanently, until you claim all its rewards.
Reaching Collector Stage I and Collector Stage II will give you the option to access a new UNITE Fund with even better rewards than the last one. However, you need to complete and claim all previous UNITE Funds for new ones to become available in the shop.