Okay, let's try it this way.
You get a water sample from a customer. From customer A, you get a quart. From Customer B you get a liter, from customer C you get 500grams.
You need 4grams to do your test.
You take a sample of what was provided to you.
You need a sampling procedure something like:
"The as recieved specimen will be shaken by hand for no less than 10 seconds, then allowed to sit for no more than 2 minutes.
A 100ml glass pippette is inserted approximately to the halfway point of the specimen container and at least 10ml of sample is retrieved.
From this retrieved sample, an amount of 4 grams (+/- 0.1gram) is weighed into the test cylinder."
And on and on...
This is your sampling procedure for a certain class of liquids for which it is appropriate. For another class with heavy suspended material you may define the sampling method differently...to ensure that your sample is representative of the stuff that showed up at your door.
I bet (hope) you already have these procedures in place...you just need to write them down.
You need the sampling plan, and sampling procedure, so that some knucklehead doesn't try to sample a sedimented suspension by pulling a sample from the top supernatant liquid that is not representative of the material received, then reporting to your customer that his cadmium contaminated water is just fine. This is also why the sampling procedure "shall be available at the location"...so the proper way to take a sample is available right at the work station.
Don't be confused that the stuff showing up at your door says "sample"...
to your testing, the stuff that shows up at your door is the population. You are taking a sample from that population.
How do you do so? That's the plan and procedure it's talking about.
How was that one? Any clearer?