The 'linking' feature allows you to take the previous report information with you when completing your report, so that you can see what you said last time for comparison purposes.
You should start scheduling your appointment as normal, and then when selecting your report template you should select 'Yes' in the 'Link to Previous Report' dropdown.
Next to the template on the right, you will see information showing you what report you are linking to, so that you can make sure it is the correct report and template.
Please note: that the report you want to link to must be marked as 'final' in the system, plus you must use the same report template so that you are able to compare exactly like-for-like.
Once the appointment is saved, you can sync it to the app. Here's a video that will show you more about how complete a linked report in the app: