What does the "work item" stand for in FPGA?

OH ,thank you! I got a lot.

In addition, when i get the device info of "de1-soc" ,the "cl_max_compute_unit" is 1. However, as we know the phrase"__attribute(num_compute_unit())" can be used to set the num of used compute_unit(). The resource usage is surely changed when i increased the num_compute_unit, though with a little performence decreased. So it puzzled me that if there is a restrict with the num_compute_unit for FPGA ,or the num of compute_units (work groups) just depends on the kernel code and the phrase"__attribute(num_compute_unit())".