What it's good for?
The external ID is used to uniquely identify the tree element for integration into third-party services. An external ID can be created on portal in user interface in section API. This The External ID you can use be used in your GraphQL queries/mutations in Survilla cloud API.
...
Adding
...
an External ID
...
In the top right corner, press the button Avatar with the username you are logged in with.
Open the API tab.
Select the External ID application interface.
...
4. Press the “+ Add identifier” button.
...
5. Click in the Item text box to open the directory structure window and select the folder or element to which you want to assign an external ID.
6. Fill in the External Identifier and press “Save”.
External ID is created.
Then you will see in a list of created External IDs.
TBD - https://system2.netrex.cz/kb/en/topic/External-ID-settings 
Editing an External ID
Press the button “Edit” next to the corresponding external ID.
Edit the necessary parameters.
Confirm with the “Save” button
...
Deleting an External ID
Press the button “Delete” next to the external ID you want to delete.
...
Example
...
Examples of integrations
Integration with Winshop - you can record billing action, etc
...