The Russell Microcap index is made up of the smallest 1,000 companies in the Russell 2000, PLUS up to an additional 1,000 even smaller companies that are not included in the R2000. As per your chart, the Russell Microcap included 1,491 companies last year, so only 491 smaller companies were not included in the R2000.
The fact that BSGM appears on both the R3000 and the R-Microcap additions lists indicates that it should be included in several of the Russell Indices in 2019:
R3000 (Made up of the R1000 + R2000)
R2500 (R3000 minus the top 50 largest)
R2000 (R3000 minus the top 1,000 largest)
R Micro (Smallest 1000 of the R2000 + up to 1000 smaller companies)