But "fully detailed" to me means that it is an exact replica of the 1:1 engine. So it would have brackets for alternator, AC compressor, power-steering pump, etc., oil and transmission dipsticks, etc. Most modelers do not include all those details in their miniature plumbed engines. Like others said, "plumbed" means it has some hoses and tubing/pipes modeled.
And as a side note, when you configure an Ethernet interface on a computer, you are also "plumbing" it. At least in technical lingo.