I've shared a blog with tips to help you Increase efficiency going offline and syncing in Collector. It covers the Advanced Offline Options that let you, as the map author, specify if your users should download all features and their attachments, just features (without attachments), or nothing (no features) when going offline and syncing. This is just for the features you are retrieving from the server. No matter what you set in those options, the edits made in the field (features and attachments) are uploaded to the server on sync. Give the blog a read, then come here to discuss it.
Thank you for the informative blog posting. This is valuable information for our deployment of our projects for Collector.
One question: At the beginning of your article, you state "By default, all features (editable and read-only) and their attachments are taken offline...". It was my understanding that all layers in a Collector map needed to be editable in order to show up in Collector and, more importantly, in order to be taken offline. Can you please clarify what you mean by "read-only" data layers and what settings are used for this type of data layer.
Thank you for sharing with the community.
Hi Joe! For a map to show up in Collector, there needs to be at least one editable layer. However, you can also include read-only reference data. Check out the "Reference data" section in Supported data—Collector for ArcGIS | ArcGIS for details. While some reference layers are only supported online, the majority are supported online and offline.
A bit more info about what is required for offline data is in the "Data requirements" section in Prepare maps for offline use—Collector for ArcGIS | ArcGIS. I think the key to your question is that feature layers don't have to be editable, but do have to be sync-enabled in order to work offline.