In the Details Panel for each file in your Mendeley Library, you will see a document "Type" menu at the top (see the top right of the screenshot below). Each type has a different set of fields that it displays in the panel. For example, a book will have a field for an ISBN but a journal will not (Instead, it will have an ISSN). Both will have Title and Author fields, but the other fields displayed will depend on the document type, and required details for citing this document type.
If you would like a new field to show up for a specific type of document, you can customize the fields. For example. add a Date Accessed field to by going to Mendeley Desktop > Preferences > Document Details [On a PC go to Tools > Options > Document Details]. Select the Document type from the dropdown menu, and then check the box next to the name of the field you want displayed (see screenshot below).
Tip: When adding new documents, you should always make sure the Type selected matches the type of document you have added, e.g.: a journal article, a book chapter, web page, or thesis, etc.
A "Watched Folder" is a folder from which Mendeley automatically uploads files such as PDFs into your Mendeley library. You can set it up by going to Mendeley Desktop > Preferences > Watched Folder [On a PC go to Tools > Options > Watched Folder] and putting a checkbox next to the folder in your harddrive that you designate as the one for Mendeley to "watch." In the screen shot below, I have named my folder "Mendeley Watch Folder" and placed it on my desktop.
Once set up, any file you add to the folder will be imported to your Mendeley library and found under "Recently Added."
Tip: Although I keep my Watched Folder on my Desktop, I clean it out periodically to avoid overtaxing the virtual memory of my computer. Mendeley creates a second copy of your file elsewhere in your harddrive, so deleting files from the folder will not remove them from your library.
Mendeley will create a back up folder for your files in your hard drive. Anything you import into your Mendeley Library will be duplicated and saved. To view or change your settings, go to Mendeley Desktop > Preferences > File Organizer [On a PC go to Tools > Options > File Organizer]. The Default location is in your Documents folder, but you can change this by using the 'Browse...' option under Organize my files.
In addition, you can choose the way your backup files are named by checking the box next to 'Rename document files.' Drag the fields such as Author or Title to the lower box to have them show up in your backup file's name. See the Details panel in the box above, Customizing Document Types for Citation Templates, the field Files shows the files in this library are formatted by Author - Year. pdf, i.e.: Gierdowski - 2019.pdf.
Tip: Customizing how files are named can be helpful if you ever need to locate a file quickly in your hard drive.