You can find this option within the Layer Menu and when ticked, ensures that the activated, original Partial Image is always located within the first repeat.

Even when dragged outside the repeat, the active Partial Image will always return to the corresponding position within the first repeat and is marked by grey, circular nodes.

Ensuring the original Partial Image is located within the first repeat minimises the risk of potential unwanted cropping of the shapes when flattening the file.