Why is there a hash value at the end of this element ID? I've created an entityreference to a content type that is just a title and an image, and set the storage settings to unlimited.  Now, when I'm on the edit form and I click the "Add Another Item" button, the ID of the input that appears is: edit-field-photo-1-target-id--nOP2UjZ0zuI

Why is that --nOP2UjZ0zuI at the end, there?  It makes it annoying to select, even though I'll be able to search for an ID that starts with edit-field-photo-X-target-id as a workaround, but what does it do?
Here is my guess:

In HTML ID's are suppose to be unique. Since you're adding another item, the ID cannot be the same. Thus, the --nOP2UjZ0zuI is probably some random generated string so the ID is unique.