As defined by Google, Assisted Conversions are:
“the number (and monetary value) of sales and conversions the channel assisted. If a channel appears anywhere—except as the final interaction—on a conversion path, it is considered an assist for that conversion. The higher these numbers, the more important the assist role of the channel.”
In simpler terms, assisted conversions are the interactions that a customer has with a website leading up to a conversion, but not the final interaction. These Assists are assigned a value based on the transaction they contributed to in order to help assign value to their contribution. It is also important to note that Assisted Conversion values can be higher than total Conversions. As stated by Google:
“Assisted Conversions for a given channel counts all conversions in which that channel was a non-last interaction. Assisted Conversion Value is the total value associated with those conversions. As as result, double-counting can occur across rows or across assisted and last conversions. If two conversions occur for a single person, any channel interactions which were in both conversion paths count toward both conversions.”
As an example: mcf:basicChannelGrouping==Paid Search will provide any assisted conversion for Paid search.