Offset
Offset is the distance from the centerline of the rim to the mounting surface. More offset moves the rim in, less moves it out. So if you go from 50mm to 40mm the new rim will be 10mm farther out than the old rim. 25.4mm equals 1 inch.
The problem we will face when offset is less is tyres hitting the fenders not the inner wall or the absorber / springs ( those with adjustable asb can use rims with less offset by making the car higher to prevent the tyre from rubbing the fender). The only thing is ur car might look like a 4 wheel drive, where the left and right tyres has a broader base, for some they like it that way.
When the offset is more that the stock, this will be a problem for all. because it will hit the inner walls and the absorber. There is no way to correct it.
I think is nice to get a rim with lesser offset but not too less. For those who are using spacer, plz dun use it unless u got a custom made spacer which fit it properly, not those u get from tyres shops. If u want ur wheel to be out get a rim with low offset. then match with the proper tyres to prevent hitting on the fenders.
Just my 2 cents.