Hi @vikash_adobe_v
Auto allocate automatically drives users to the winning experience, which is decided by the goal metric you set up. for example, if your goal metric is clicks on Add to Cart, and in challenger experience, you have changed the font size of the button text.
Now, irrespective of whether the user noticed that change or not, target would just track the number of users getting converted (i.e. clicking on Add to cart) from exp. A VS exp. B and basis that the winner will get more user allocation. Along with this some amount of traffic is reserved to explore both experiences.
If your goal metric is A2C clicks, then obviously your changes should be around the A2C button which will be noticed by users. sometimes some minor changes on the page could affect a user's behavior drastically even if the changes are unnoticeable.