I watched twice the lesson about importing reference materials, however I have some difficulties getting to understand it. Now I understand the size of pin is 38cm and I understand that 1.9 but I cant quite get that 3,8BU…why not 38? I believe I ll be ashamed when I read the answer but at this point I just cant get an answer…what…decimeters? Why? Still not getting what is the exact BU…when set to default NONE option on length under unit section in properties panel…Sorry for probably stupid question… 
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The teacher chose the scale of 1 BU = 10cm because it’s easier to work with. That is arbitrary. You could totally decide that 1BU = 1cm and then scale the pin to 38 BU but that’d be a HUGE pin on the screen! Imagine that, if the pin is that size in the scene, and you had to build a scene around it, every other thing would have to be even bigger so that the pins look small.
And then, if you start making everything huge in blender, you may start to have troubles with zooming in and out of the scene.
So, the teacher decided that the best option was to make 1 BU ten centimeters.
Blender unities can mean anything. You can decide that 1 Blender Unity is equal 5km, 6dcm, 10m, 100cm… Anything really. It’s an imaginary unity.
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Thanks that was the missing piece in my understanding, bacuause I sort of understood that meters are the unit (I believe even in the lesson about scaling he placed meters and length to none, so that got me confused, but now I think I got it…and since he always talks that on start up file, or load up file the cube is 2BU, now I figured why that scaling in sphere too… Thanks!!! 
And yes I am ashamed now hehehe
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Never be ashamed to ask for clarification! The only stupid question is the one unasked.
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