Polaris type electrical connectors terminology
Your wire is #8 AWG, copper since it's a 40 amp circuit. So you need a connector that will fit that wire. The sizing on the connectors states what wires will fit in the individual holes in the connector. So in your question you have a connector that is rated for "1/0 - 14 AWG", it will handle a 14 AWG to a 1/0 AWG wire. Since your wire is a 8AWG, this connector would work. A sample wire chart is shown below.



Since these are expensive connectors, you want to get the smallest one that will accommodate the wire sizes you are using. 
There are other connectors available so you might want to ask another question about this and explain what you're connecting and you might get some better ideas.