Incremental copy is a migration option in your Operation Mode settings that will replace an item at the destination only if the date modified field is older than that at the source, and copy new items from the source that don't exist at the destination. This aims to ensure that you are not overwriting newer files at the destination.
This article will take you though the default behavior of Incremental Copy, so you can decide if it is the right option for your oncoming migration project.
If your destination library does not have same folder hierarchy as your source, Sharegate will be unable to determine the items are a match, and will create a copy at the destination.
Large lists or libraries
Incremental Copy is not designed for large lists or document libraries. In this mode, Sharegate crawls the source and destination to read your content in batches of 1000 items at a time to determine what needs to be copied. So if you have more than a couple of thousand items, Sharegate will need to make many calls to obtain the data. This can result in your migration job going slowly or being unable to start altogether due to performance issues or throttling (on Office 365).
If you are migrating a large list or document library, we recommend using Copy and Replace, setting your Filter Options to enable Time Range Filter, and setting the time frame to limit your migration to elements created or modified in your chosen range.
Documents that were deleted at the source
Incremental Copy is not meant as a full synchronization option, and will not identify or delete items at the destination that were deleted at the source. This avoids any potential data loss during the migration process. You will need to manually delete those items once your migration is complete.
Incremental Copy Structure
When using Incremental Copy in a Copy Structure migration job, Sharegate will only consider List Items and Documents. Any structural element is still rebuilt based on the source since SharePoint doesn't keep an accessible record of your library, list, site changes, and settings.
Incremental Copy does not check for permission changes from the source to the destination. It only validates the last modified date when copying.