Exact Match Impression Share metrics:
- A) are available for both Search and Display Network campaigns.
- B) summarize impression share statistics for all keywords currently set to exact match.
- C) calculate impression share as if all keywords were set to exact match.
- D) are only available at the account level.
Explanation: Exact match impression share is the percentage of impressions that your campaign received for searches that exactly matched your keyword, divided by the estimated number of exact match impressions you were eligible to receive.
You can read more here: https://support.google.com/adwords/answer/2497703?hl=en