The advanced search options look like this:

Enter your search term in the search box. You can limit your search to a particular field.

You can add further search terms with the add new criteria button:
•    Use And to add a second term and return only results with both terms
•    Use Or to add a second term and return results with either term
•    Use Not to add a second term and return only results with the first term and not the second

You can also limit results by:
•    Repository: We host the archive catalogues of some of our heritage partners on this site (see the full list here). Use this field to limit your search to one of these hosted archives.
•    Top level descriptions: Click here or start typing to view a drop-down list of all collections. Select a particular collection to limit the results of your search to the contents of that collection

You can also filter results by:
•    Level of description: Use to limit the results to a specific level of description
•    Digital object available: Use to limit the results to entries with a digital file attached
•    Finding aid: We don't make use of this feature
•    Copyright status: Selecting Image access restricted will find only those items where the attached digital image is not viewable for copyright reasons. Selecting Unknown will filter out those items where the attached digital image is not viewable. Leaving the filter blank will show all items.

You can also filter by date range - for which you need to select overlapping or exact.

You can find more information on the AtoM website.