UI Identifier is the Testsigma terminology that we use for elements on your Web Pages or App Screens that we would like to interact with or automate in the Test Steps.
In the below page, Full name, Email-id, Age Group, Gender e.t.c are all elements in the web page. In order to interact with any of these page Elements, we have to create a UI Identifier for that element and use the UI Identifier name in Test Steps.
Let us consider the situation where we are going to automate the task of entering 'John' in the 'Full Name' field.
Firstly, we will create a UI Identifier named 'fullname_field'(you can provide any meaningful name).
Once the proper name(First Name, Last Name, Email etc.. ) is given to these elements, next step is to fill the Attributes/ properties, XPath or CSS selector for these elements.
The necessity for these attributes is that when we run this step through automation, we would need certain Attributes/Properties, XPath or CSS selector of this First Name Field to locate/access this field and enter the value “John”.
We can find value of the UI Identifier in a couple of ways as explained in the following article: Different ways to create/update UI Identifiers