Detailed explanation of some options:
Starting Resources (in %):
Default starting resources for various difficulty levels are following:
Rookie: 200 Tech, 800 Materials 400 Food
Veteran: 150 Tech, 600 Materials 300 Food
Hero: 100 Tech, 500 Materials 200 Food
Legend: 80 Tech, 400 Materials 200 Food
You can increase (by setting number higher than 100) or decrease (by setting number lower than 100) amount of starting resources via mod settings.
Tactical difficulty modifier:
When you start a new game without this mod or with this mod and 0 in tactical difficulty modifier on for example Veteran you will get Veteran difficulty for both Geoscape and Tactical.
Changing this number allows you to have different difficulties for Geoscape and Tactical.
Geoscape difficulty is going to be the one you choose when starting a new campaign and tactical difficulty can be modified via tactical difficulty modifier. Changing its number to 1 means that your tactical difficulty will be one difficulty level higher than the Geoscape difficulty. -1 means tactical difficulty will be one level lower than Geoscape difficulty.
You want to play the game with Veteran Geoscape difficulty but Legend Tactical difficulty. Set Tactical difficulty modifier to 2 and start the game on Veteran.
You want to play the game with Hero Geoscape difficulty but Rookie Tactical difficulty. Set Tactical difficulty modifier to -2 and start the game on Hero.
Mission threat level modifier:
Most of the missions are using threat level. There are 4 threat levels: low, medium, high and extreme.
Inputting 1 into Mission threat level modifier means that every mission that is using threat levels will be one level higher than it would normally be. -1 means that every mission will be one level lower.
If you set this number to 3 then every single mission in the game that is using threat levels will be an extreme threat level mission.
If you set this number to -3 then every single mission in the game that is using threat levels will be a low threat level mission.
Starting soldiers modifier:
Once again this modifies the default setting of difficulty level that you choose when starting a new campaign.
Default number of starting soldiers for difficulty levels:
You want to play Rookie with 12 starting soldiers: Input 6 into starting soldiers modifier and start the game on Rookie.
You want to play Legend with 1 starting soldier: Input -3 into starting soldiers modifier and start the game on Legend.
Minimum number of starting soldiers is 1 and maximum is 25.
Human population lost:
Whenever the human population is lost it is modified by the % number you input here.
If you set it to 1000 you will automatically lose the game in about 2 weeks purely because of the haven starvation as its effects on population meter are ten times bigger.
If you set it to 50 then the minimum possible value for the population meter is 50% because when all the population dies it will affect the population meter by just half of the real value.