A .csv file that you must upload with your customerId list and/or custom data. The CSV file must contain always a column with the header customerId at least and your customerIds. If you want to send a comunication with variable params for each customer, the header must contain the name of every param as columns.
Example customer list
customerId
be5d3f02-f486-47e2-bd98-5128e1c4b065
be5d78f2-f486-47e2-bd98-5128e1c4b066
be5d3f02-a345-47e2-bd98-1234wqe34241
...
Example campaing with variable params
customerId,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.