Difference between PGID/PUID and GID/UID when setting up docker containers

I'm new to Docker and I've seen that some scrips for running containerized software require to specify either of those to match certain user and group. So I was wondering what is the difference between both? Examples below to illustrate.

Jellyfin uses UID/GID pair:

docker run -d \
    --name="Jellyfin" \
    --restart=always \
    --network=host \
    -p 8096:8096 \
    -e UID=1000 \
    -e GID=1000 \
    -v /media:/media \
    -v $HOME/docker/jellyfin/config:/config \
    jellyfin/jellyfin:latest

While Calibre-web uses PUID/PGID set:

docker create --name=calibre-web --restart=always \
    -v ~/books:/books \
    -v /etc/localtime:/etc/localtime:ro \
    -e PGID=1000 -e PUID=1000 \
    -p 8081:8083 \
    technosoft2000/calibre-web

While looking for info I even saw some random github repos that changed from the former to the later at some point; what's the difference between both options?