Please help me understand

The below works

location /stats/ {
proxy_pass http://example.com;
}

or

location /stats {
proxy_pass http://example.com;
}

or

location /stats {
proxy_pass http://example.com/stats;
}

or

location /stats {
proxy_pass http://example.com/stats/;
}

or

location /stats/ {
proxy_pass http://example.com/stats/;
}

But this does not work

location /stats/ {
proxy_pass http://example.com/stats;
}

Smlly when stats is an upstream

This works

location /stats {
proxy_pass http://stats;
}

but this does not

location /stats {
proxy_pass http://stats/;
}

What difference it makes when we have uri in the proxy_pass?

Regards
Amod