This is an experimental feature, it is not available for all accounts.
If you are on the Boost plan, you can use this feature. The important pre-requisite is, that your User Tracking Mode must be set to Full sync.
This feature allows you to change selected properties for multiple users. This is not the batch upload feature, but batch update. The selected properties will be set to the same values for all selected users.
The main use-case is if you want to display Product Fruits content to a group of users but you know only usernames or e-mails and these users don't have any other common property. You can upload the list of users, create a new custom property and set it to some value. You can then use this custom property in the segmentation.