This file is used in random encounters. When traveling between areas on the worldmap, if a random encounter is triggered the engine compares the party's reputation against rows in this file. Based on the percentage change associated with that row, the party may be rerouted to the base random encounter area for that area group ("AR10" e.g. if the random encounter area was anywhere in the AR5700 - AR5799 range, the party instead is taken to AR5710).

The first column contains reputation values.
The second column contains a percentage value.
In BG1 TotS we have:

2DA V1.0
0          100
1          100
2          100
3          75
4          50