View-through conversion tracking provides additional information related to the value of your Ad Exchange ads. This feature provides a measure of the number of online conversions that happened within 30 days after a user saw, but did not click, an Ad Exchange ad. View-through conversions are tracked via a cookie that is placed on a user's computer; conversions are reported only for Ad Exchange ads that run on Ad Exchange sites.
Pass-back line items are used with third-party advertisers for a number of reasons. Primarily, pass-back line items return a house ad or other remnant ad within DFP when the third-party doesn't have an ad to serve or when this ad doesn't meet the minimum CPM agreed upon with the third-party.
Impression share (IS) is the number of impressions you've received divided by the estimated number of impressions you were eligible to receive. Eligibility is based on your current ads' targeting settings, approval statuses, bids, and Quality Scores. Data is available at the campaign and ad group levels.
In general, non-guaranteed line items fill your site's unsold inventory when you haven't made any contractual impression agreements with your advertisers. Non-guaranteed line items include: network, bulk, price priority, and house. They aren't considered booked inventory by forecasting.