Learn more about External Sharing functionality in the Microsoft documentation.

Title
The first part of the business rule lets you define a title for your rule and select the event on which it operates. The default title is the standard FlowForma rule name; change it by deleting the text and entering your own. In the event field you determine when the rule is executed. For a flow this can occur when the form is loaded, started, or completed. For a step this can occur when the step is started, saved, or completed. For questions it can occur when the question is updated.Rule condition(s)
The second part of the business rule lets you set conditions. These are not required, but if entered they control when the rule is executed in relation to other items in the form.Rule actions
- URL to folder or file: Select the question that will contain the URL to the file or folder you want to share. You can also paste the direct URL into this field.
- Set generated link to: Select the question that you want to set the generated link to.
- Stop execution: if stop execution is enabled and this rule is executed, no other rules are executed.
- Enabled: rules aren’t saved to a list like questions and steps are, so if you want to stop a rule executing without deleting it, toggle this option to enable or disable the rule.
- Enable audit: if checked, a record is added to the audit trail when this rule is executed. This option is available when the Compliance feature pack is activated.
Note: to use external sharing, you must use a modern SharePoint document library, and external sharing must be enabled on your site collection.

