Lets say you have the horizon settings set to great strategies from January through October a period of 10 months.
With Out of sample set to data_start_percent = 0 and data_end_percent = 100 then EG will use all 10 months to create strategies.
Now with Out of sample set to data_start_percent = 10 and data_end_percent = 90 then EG will use 8 months from February through September to create strategies with January and October not being used in creating strategies. These two months are not in the sample being used to create strategies, therefore, they are out of sample.
Test it and while EG is creating strategies you can view the range of dates it is using.
Alan,