Hi @rut7 ,
Consider the below use case,
You have 6 recipients in query output
| recipient |
Country |
Gender |
| A |
INDIA |
MALE |
| B |
INDIA |
MALE |
| C |
INDIA |
MALE |
| D |
INDIA |
FEMALE |
| E |
US |
FEMALE |
| F |
US |
FEMALE |
Here 4 recipients belongs to INIDA and 2 belongs to US
and 3 are male, 3 are female recipients

If you have split with 2 subsets.
Subset-1 : country = INDIA
subset-2: Gender = Male
If you don't enable 'overlapping of output population', in subset-1 output will be 4, and subset-2 output will be 0. Though you have 3 male records, but those records now went into subset-1 transition as they belongs to subset-1 criteria as well (country as INDIA). So those records wont come in subset-2 transition.

But if the campaign scenario is that you want output like, 4 records in subset-1 (India) and 3 records in subset-2 (Male), then you want to enable 'overlapping of output population'.

Hope this example clear your question