Tinned Copper Tubing, What the heck is this now?

Need to hook up a natural gas fire log inside my fireplace. It says to not use regular copper tubing but 'Internally tinned copper tubing'.
What is this stuff and how can you tell if you have it? Is there a marking on it that says its tinned?
Ben
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What, you can't tell by looking?
If it's tinned, it'll be tin-colored (similar to stainless steel). If it's not tinned, it'll look like copper.
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--------------- http://www.lni.wa.gov/forms/pdf/622046a0.pdf#search ='tinned%20copper%20tubing'
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