As explained here, the upload process can support the addition of constituent and activity records together or separately.
In the simplest case, one is uploading a file of constituents with no activity data explicitly in it and appending a default activity record at the default setting stage — for example, if all of the constituents uploaded were signers of a petition.
It is possible, however, to supply activity data in the input file itself. In that case, each activity record in the file needs to identify the issue that it is associated with. You can do this two ways — either by mapping an input column to issue number (which is the WordPress post ID of the issue, visible in the URL for editing the issue) OR by specifying the exact title of the issue. If you map issue number, it has to be a valid existing number. If you map title, however, WP Issues CRM will, at the default stage, offer you the option of creating new issues out of unmatched titles. The content of the created issue is unpredictable in a file with multiple identical title lines.