Dictionary

Investment Grade

A bond is considered investment grade or IG if its credit rating is BBB- or higher by Standard & Poor's and Fitch or Baa3 or higher by Moody's. In general, these are bonds that are judged by the rating agencies as likely to meet payment obligations of interest and principal on maturity. In the Capital Market Data model we use IG in League Tables and Sector Flow Statistics.

Wikipedia

To check who leads in Corporate IG:

To understand Flow Statistics check table 7