The creative element in VAST, contains the media files (video files, companion banner files, etc) for the ad unit. The ability to include multiple media files for this element is very useful. An advertiser can include different video file formats that are optimized and display best on different devices. For example some of the most common video file formats are mp4, webm and 3gpp. Another file format that is sometimes sent is flv, but very few mobile devices can support flash, so this format will not work on many of them. The most common and universal format that should work on all major devices is mp4, so it is important to always include this file format.
Sometimes different sizes of each file format are also included. It can be useful to give the video player different file sizes so it can choose which one will display best on the device. Also smaller dimensions, if they still display properly, can help the video file to download faster and thus provide a better user experience.
There are numerous benefits to providing multiple video file formats and dimensions, but there are a few things that the advertiser and publisher should be aware of.
For the advertiser, you need to make sure that you include at least one universal format (ie mp4) and size that can display properly on all devices. Failure to do this can result in a poor user experience or the video not playing at all. The other potential downside to lots of different creative files and sizes, is that it takes the VAST XML longer to be sent to the publisher and requires more processing time for the video player to identify the proper creative to display. The additional time to send and process this data is typically negligible, and if there are only a handful of creatives then it should not be detectable, but if there are a lot of creatives, then there is the potential for some latency.
For the publisher, you need to be aware of what video file formats your video player can support along with the logic of how the media file is chosen. The creatives are sent in a linear fashion, with one creative after another. When the video player goes through the sequence of choosing the proper media file to display, it sometimes will play the first media file it finds in the list that it supports regardless of the size of it. This can be problematic if it is small in size and does not display well.
Another potential issue is that the video player may choose the first media file in the list, regardless of whether it can support it or not. This is not common, but some custom video players use this behavior and it results in many videos not playing due to incompatible file formats.
The modern, mainstream video players should select the proper video file format and size without any problems, but if you have a custom video player then it is something you need to check on.
The publisher does not have any control over the video file formats they receive in the XML, but they should be aware of the capabilities of their video player so they know what to look for when troubleshooting poor use rate of returned video ads. Knowing what file formats your video player supports and the logic it uses to select the media file is useful information to provide to your advertising partners, so they can structure their campaigns to deliver ad units you can use and hopefully limit the mobile video creative errors you experience.