The UID indicator (blue light button on the right below the power on/off button) is to help you to physically identify a server in your server room (in a rack or in your datacenter).
You can turn the UID indicator On or Off as follows by click on the iLO circle icon. You will see the button turning blue, indicate on the server that the UID light is on.
Documentation
https://www.arubanetworks.com/techdocs/AFC/610/Content/afc61olh/hpe-ilo-act.htm
https://community.hpe.com/t5/proliant-servers-ml-dl-sl/uid-light-is-on/td-p/7007160