A global variable in LabVIEW FPGA acts like a global variable in normal (non-FPGA) LabVIEW, but with the tradeoff that you don't get any control over how it's implemented on the FPGA. It's a good choice if you're familiar with LabVIEW but not FPGA and want to get your code running without worrying about the low-level details.
A memory item, whether implemented in block memory or in LUTs, acts like an array where you can access individual elements. Although a global variable or a register can contain an array, it is always accessed as a single unit.
As an example comparing a register or global variable versus a memory item, consider a circular buffer, where on average the read and write rates are the same but sometimes you write several items before doing a read, and sometimes you read several items between writes. A memory item is a the right choice because you can maintain independent read and write addresses, and avoid accidentally overwriting data so long as the addresses don't catch up to each other. With a register or global variable you'd need to read the entire array out, update the relevant elements, then store the entire array again, which would be more complicated and use more FPGA logic.
