Fill Density
From MatterControl Manual

Migrating: For MatterControl 2.0 the MatterControl documentation is moving to matterhackers.com/mattercontrol/support/. 
Function
The amount of infill material to generate, expressed as a mathematical term specifically, ratio or a percentage.
Parameters
 percent (%)
 ratio
Can be zero.
Presets
 Light : 10%
 Standard : 30%
 Heavy : 90%
 Custom : userdefined (can be % or ratio)
Example(s)
%
Using a percentage. In this case, the infill will be 37% of the total area designated for infill.
ratio
Using a ratio from 0 to 1. In this case, 0.89 translates to 89%. Actual infill will be 89% of the total area designated for infill.