Indirect waste receptor

What is meant by a Indirect waste receptor when talking about a TPR valve.Does it mean that it is not connected to the drainage system of the home?
I inspected a home yesterday that had the discharge pipe routed horizontally with 3 -90 degree fittings before going vertical into a trapped standpipe with also other drain lines in it such as water softener. I am recommending they get re plumbed to go straight down to 6" from the floor but the is no floor drain. Nachi graphics show it going into a bucket. Is the bucket the indirect waste receptor & if so if the pipe was 6 " off the floor inside the bucket, couldn’t that be a siphon situation also. Wouldn’t the end of the pipe have to be 6" above the bucket to create the air gap?