Each row represents 1-many Filters or Value Fields (for simplicity's sake, both filters and value fields will be referred as elements). 

Ordinary elements

The elements are grouped by data, the element works with. 

An example of this grouping is filters working with the keywords Meta data field:

The The Details panel for this filter:

In this grouping, the two filters are a FreeText filter and an InList filter. 

Group

An exception of this grouping is when a filter is a Group. In this scenario the group's filters are grouped together even if the data, which they work with, is different from one filter in the group to another. 

The reasoning behind this is that the Conditions in the group will only make sense if the group's filters are placed together.