Global Grouping Mode: With this option, it is possible to treat the dimension values as either ‘local’ to the current upper dimension level in the chart, or ‘global’ to be evaluated across all instance of the upper level dimensions. This option is only applicable to the second or lower level dimensions on a chart.
Whenever you set dimension limits, they could be set up taking into account the values on the first expression (largest values) or order of the records in the field that is being used as dimension (first values or last records)
Now, in Aveeeeeee7en's sample, you see that you can either:
See which are the Product Families that are top performers in their particular region (Global Grouping Disabled)
Or see which Product Families are your top performers (Global) and check their performance by region (Global Grouping Enabled)
As you can see, in "Global Grouping" the product families are always the same in all the regions (since these are your top performers at global level) but when this option is disabled, you will see that there are some Product Families that are have a better performance in some particular regions.