Split Starts -vs- Split Grids
As of June 1, the language in GCR 6.5.2.D is replaced (see June Fastrack, p.6):
6.5.2.D Split Starts
1. Provided each segment is properly formed, each will receive a separate green flag.
2. If the first segment receives a green flag, the race is considered to have begun for the subsequent segments when
they cross the control line, regardless of the flags displayed by the starter.
3. The flags displayed by the starter have their normal meanings.
4. If the first segment receives a green flag, but on track safety conditions require an immediate full course yellow, cars
in subsequent segments should fall into single file grid order and make every effort to safely catch the back of the first
segment.
5. Drivers in each segment shall not improve their position until their respective green flag is displayed. Jump starts may
be penalized.
It's important here to distinguish between split starts and split grids.
A Split Start is defined in the GCR: separate segments, each usually with its own pace car, and each taking its own green flag. If the first segment gets a green, the second segment will also get a green, but not until it reaches the Starter. If segment 1 messed up, Start may also display other flags (Yellow, FCY, etc.) When each segment takes its green, racing commences throughout the segment.
A Split Grid is not defined in the GCR, and is therefore an informal local arrangement. Typically, classes within a segment will group themselves separately from (but close to) the rest of the segment. When the segment is shown the green, racing commences throughout the segment (as above).
For the race under discussion, there was both a split start (segments 1 and 2) and a split grid (segment 2 split into two). When segment 1 took the green, it commenced racing, but segment 2 did not. When the first car in segment 2 took the green, racing commenced throughout segment 2, including both split groups.
As another poster mentioned, these are complicated processes, with many moving parts. Combining a split start and a split grid adds to the complexity. It is critical that all players be on the same page.