Depending on the system design and the configuration, it is possible to mix videos of different aspect ratios in one timeline. As an example assume a linear television production in 16:9 using videos recorded using a camera. This production now should be enriched with content from a smart phone shot in 9:16.
Having both kind of files ingested to the system they can easily be added to the VidiEditor and can be arranged while seeing the result in the timeline player.
Please not that the player automatically scales the mobile (9:16) video to larger side depending on the defined timeline output format (see General Configurations [VE OG]).