The legacy mode is only relevant to you, if you want to run pixolution flow 3.x with an old 2.x index.
With pixolution flow 3.0.0 we introduced new image descriptors, which requires reindexing the complete image collection to calculate the new image descriptors and store them in the Solr index.
This can be very time consuming and might be a barrier to update pixolution flow from version 2.x to 3.x. Additionally, pixolution flow 3.x requires a Solr update which makes it even harder to update if you have to update all at once.
With the legacy mode you can run pixolution flow 3.x with indexes created by pixolution flow 2.x. It is also possible to create legacy image descriptors for new images if you like to preserve the old pixolution flow 2.x indexing behaviour.
This way you can update Solr and benefit from bugfixes and update to the new image descriptors later on.
The following sections describe how to enable the legacy mode with pixolution flow 3.x.