Which conversion metric can give you more insight into how your ads drive conversions on mobile phones, computers, and tablets?
As an advertiser, you want to track all the conversions that your AdWords campaigns drive, including those that begin on one device or browser and end on another. By using the “Include cross-device conversions” setting, you can include these conversions in your “Conversions” column and factor them into any automated bid strategy that you’ve set up.
Cross-device conversion is a part of “All Conversion”. People learn about your business across multiple devices and screens. All conversions helps you attribute your conversions — online, in-store visits, or over the phone — to the right campaign or ad group.
In order to measure cross-device conversion statistics, part of “All conversions,” Google use aggregated and anonymized data from users who have previously signed into Google services. This allows Google to provide advertisers with highly accurate and granular reporting on cross-device behavior without compromising user privacy. Google only show cross-device conversions when there’s enough data to make a highly accurate calculation. That means that to report this data, Google must have at least 95% confidence that the calculation is within 10% of the actual value.
Read more here: https://support.google.com/adwords/answer/3419678?hl=en