When an SSIS package uses SQL server access from sql accounts, the package can be scheduled to run in a job under SQL server agent and everything will work fine. When a Windows account is needed because you have to read/write files from the file system, then either the SQL agent account needs access to those files on a file share or you need a windows account with sufficient access rights to the files. In this last case a proxy account can be used. SQL server agent will use this account to run the package in one of the steps of the job.
How to setup a proxy account and the corresponding credentials is explained in this article from mssqltips