When the number of photos to be opened is greater than nine, the ‘Choose Number of Photos to Open’ dialog provides various options to control the number of photos to open.
The top options control whether to only open photos that are not yet marked, already marked, or all, which is useful when using this tool while control point marking.
The big buttons control the number of photos to open. When something other than all photos is selected, the first one, four or nine photos will be opened in numerical order based on their photo IDs.