What does PS Form 3600-FCM serve as?

PS Form 3600-FCM serves as the Postage Statement for First-Class Mail. It is specifically designed to provide a detailed accounting of postage used for First-Class Mail services. This form is essential for businesses and individuals who are sending mail, as it includes critical information such as the total number of pieces being sent, the postage amount, and other relevant mailing information. By accurately filling out this form, senders can ensure they are utilizing the correct postage for their mailings, which helps streamline postal operations and prevents issues related to underpayment of postage.

Other options suggest different functionalities that do not align with the specific purpose of PS Form 3600-FCM. For instance, a digital shipment invoice would pertain more to electronic shipping and tracking rather than traditional mail. Additionally, a package tracking notice would focus on the tracking of parcels rather than the accounting of postage. Lastly, a mailing list connection form would involve compiling or managing mailing lists, which is unrelated to postage statements. Therefore, the specific designation of PS Form 3600-FCM as the Postage Statement for First-Class Mail is what makes it the correct answer.