There isn't one. The market does not distinguish them, and the vendors say so themselves. Visier states that people analytics "is also often referred to as workforce analytics or HR analytics." Lattice names its product "People & HR Analytics Software," using both terms at once. One Model calls itself HR analytics software while describing its top edition as end-to-end people analytics.
The only real pattern is generational: "people analytics" reads as the current label and shows up in positioning, while "HR analytics" persists in page titles and product names. Anyone drawing a hard line between the two is selling you their own category.