How to interpret Nearest Neighbour Index (NNI) and Z-Score in Nearest Neighbour Analysis (QGIS)

I performed a Nearest Neighbour Analysis in QGIS (3.4) via Vector > Analysis Tools > Nearest Neighbour Analysis on two different point patterns (13 archaeological sites in each). I am struggling to understand what the values mean. Do the NNI for both patterns indicate that the points exhibit clustering, since they are <1? Does the Z-score for point pattern #1 indicate clustering, and Z-score for point pattern #2 a random process? What is then the difference between NNI and Z-score to understand the behaviour of the point patterns? Is there perhaps a better way of calculating these values in R?

Point pattern 1

Nearest neighbour index: 0.7810443299917054

Z-Score: -1.5102844644713787

Point pattern 2

Nearest neighbour index: 0.9892311178623155

Z-Score: -0.07428022024573287