A .csv file that you must upload with your deviceId list and/or custom data. The CSV file must contain always a column with the header deviceId al least and your deviceIds. If you want to create a campaigns with variable params for each device, the header must contain the name of every param as columns.
Example device list
deviceId
be5d3f02-f486-47e2-bd98-5128e1c4b065
be5d78f2-f486-47e2-bd98-5128e1c4b066
be5d3f02-a345-47e2-bd98-1234wqe34241
...
Example campaing with variable params
deviceId,name,title,url
be5d3f02-f486-47e2-bd98-5128e1c4b065,Jon,The perfect day,http://www.jonday.com
...
The variable params {{name}} will be replaced by Jon, the same will happen the other variable params.