​@thijsknapen​  Input Group Mode: Process When Group Changes (Advanced) was introduced in FME 2019:

Pretty messy to explain for transformers that have multiple input ports. But in essence the groups on the 'primary' input wait for their matching group on the 'secondary' input port. If another 'secondary' group arrives first, then those are 'unpaired' and don't get a match. So in the image below, the red & purple on the top row find their groups on the bottom row, but the green, yellow & final red do not.

Not really the desired results, so in this case using Group By Mode: Process At End would give the expected results.

So why bother...? What this means is that provided the groups on the primary & secondary inputs are in the same relative order, it doesn't matter if the primary feature (left) or secondary (right) features arrive at the transformer first, they will be grouped properly.

The performance advantage is that as soon as a pair of groups are matched, they can be released from the transformer so less data is cached. But... notice how the red groups are separated and so the first group of red data does not get paired with the last group of red data. If you use Group By Mode: Process At End then all the reds will be grouped. So there is a difference in behaviour. In the above example:

• Group By Mode: Process At End would result in 4 groups (one for each colour)
• Group By Mode: Process When Group Changes (Advanced) will give 5 groups (red, purple, green, yellow, red again)

Prior to FME 2019, the groups would have to arrive at the input ports in the correct order, so the reds for the primary and the red for the secondary would have to arrive at the same time, then the purple and so on. this was very hard to accomplish.

I've attached an example workspace that matches the colours:

@Mark Stoakes​ Thanks for the explanation!