VMware Cloud Director uses vmxbundles as the standard format for vApp backup and restore. When you download a new vmxbundle from the vCD tenant portal, you can later upload it to restore an entire multi-VM application stack with its original networks and security policies.
If you are looking to "download new" content related to these bundles, you are likely either trying to download the software itself or a pre-configured Virtual Appliance (a ready-to-use virtual machine). Where to Download vmxbundle download new