That key controls the limits of shared heap memory
Parallel processes often need the means to share data, so Windows provides a shared Heap (the heap is a pool of memory used for dynamic memory allocation) that multiple processes can access. Usually, Processes are completely isolated in the memory they can access, so shared memory structures allow them to collaborate.
Windows creates a System Heap, for all system processes, a Desktop Heap, for all processes running under a particular Interactive user session, and a Non-interactive desktop heap, for processes that run headless in a non-interactive session.
The syntax for the key you highlighted is:
SharedSection=[system],[desktop],[noninteractive]
All the values are in KB.
This registry key lets you configure each of the heaps, with the suggested setting increasing the shared heap available to the processes running under each interactive session from 3,072KB to 30,720KB
The setting seems safe enough; generally speaking, having more shared heap would let processes share more information, but in this case, its almost certainly done to support the creation of many processes whose threads perform tasks using shared memory.
Microsoft mentions Shared Heaps in conjunction with Direct3D and DirectX, which use them to store Textures, and also specifically mentions that shared heap can be used to map to data across multiple devices such as GPUs, and avoid marshaling of ram by the CPU.
