The number lower left, is the total number of troops in that square. The top right number is the overall average Battle rating of the troops in that square. This index represents the probablity to know exactly what a certain square contains at a certain moment in the match. More information will be obtained as the VI grows. The VI calculation depens from your nearest troops to eaach square and its visibility range.
Cities has also a visibility range that increase with their size.
You can read the details on the Bonus page. Thieves have a special visibility bonus that increases their troop base visibility range by one plus another one for each two level of the leader. For all other classes, only troops joined to the leader will gain a bonus of one square per four leader levels.