Sorry - forgot that traffic type can be a Secondary dimension in some Standard reports; have never used it. Traffic Type is equivalent to ga:medium, most of the time. I say most of the time, because GA will throw in ga:source as the Traffic Type value in some cases, i.e. "direct", or a specific value related to the medium, i.e. "paid" for a "cpc" medium.
You can play w/these in the Acquisition > All Traffic > Channels report. Look at the various Primary Dimensions adding Traffic Type as a Secondary Dimension to see where they are mostly the same. Since Traffic Type is not available in the Core Reporting API, I'd consider changing your Custom report to Channel Grouping (ga:channelGrouping), source/medium (ga:sourceMedium) or medium(ga:medium).