the answer is in the documentation. The 'identifier' can be the name of the module OR a module positions in the case you want to load more than 1 single module. So you choose a desired value from the dropdown, you don't have to look up nothing at all.
I report the text where you find the answer to your question from the documentation.
To retrieve a certain module you have to specify:
1. The module identifier, it's the real module extension identifier as running on Joomla! such as
'menu' or 'mod_menu'. As an alternative you can select a module position and leave the
module title field empty. In such case all Joomla 'menu' modules assigned to that position will
be loaded. This is particularly useful if you have a multilanguage site with multiple menus.
2. The module title, it must be specified exactly as you assigned it to the module instance and it's
case sensitive, for example 'Main Menu'