Append Snapshot is a great way to add small bits of metadata for any reason. Maybe you took a large snapshot and it took a while to complete. You don't want to take that time to grab the new pieces of metadata, this is a great use case for Append Snapshot. You can add those new pieces of metadata to the existing snapshot that was taken previously.
You can get there by right-clicking on the Snapshot icon you want to work with, or you can select the Snapshot icon and click on the Options menu in the top left.
When you first open the Append Snapshot screen you will see 3 columns. From left to right they are "Current Snapshot" "Deployment History" and "Setup Audit Trail". The Current Snapshot column shows all of the general information related to the targeted snapshot. The Deployment History column is there to outline if deployments have happened since your last snapshot, and the Setup Audit Trail is there to outline anything that may have been added, deleted or updated in the org since the last snapshot.
From there, you can click next, which will take you the the Main Assets tab. Main Assets include Packages, Profiles, Permission Sets, and Custom Objects. Within each column you'll either see options in regular text, or bolded. Bolded options are those that are already included in the snapshot. This allows you to choose of the asset types that are not already included, which ones you want to add.
Note: In any of the columns you can use the quickfind box at the top to search for a specific piece of metadata.
The remaining tabs and their columns are outlined as follows:
|Other Assets||Folder Assets|
|Roles||Content Assets||Page Layouts||Dashboards|
|Groups||Custom Metadata||Apex Classes||Reports|
|Queues||Static Resources||Apex Pages||Email Templates|
|Custom Object Translations||Flows||Other Assets||Documents|
Finally, you will go to the last tab, Append Snapshot. On this tab, you will see everything you have included to add to the snapshot. All thats left to do is click the Append Snapshot button in the top right. Depending on how much metadata you have included will dictate how long this will take, however, it should take less time than the original snapshot.
Note: keep in mind when appending a snapshot what metadata you include. It will only add what you select. For example, lets say you add a new custom object through Append Snapshot, this will add the object, but it will not add the related field level security information to the snapshot, since the profiles/permission sets weren't included.
Append Snapshot is a great tool for adding one off pieces of metadata when necessary, but its not great if you are trying to gather a lot of new metadata. At that point we recommend taking a new snapshot.