units of filter option in lidR?

When using readLAS with the lidR R package, the useful filter option allows to specify options like -thin_with-grid to light the poitn cloud to be loaded. I have 2 questions regarding the units used in this tool:

1. What are the units of the distance in this specific filter? Units of the point clouds or international standards?

2. If answer to 1. is "units of the point clouds", it can be slightly tricky when you use imperial units. So is there a way to specify units? If not, I guess we have to convert value to units of the point clouds…