1st time Apple Music. Wait, what happens with my library? Merging

So 1st time with Apple Music. Just got added to cuzzo's family plan. Never used it before.

When I signed into my MacBook music app (I have my main iMac at home with full library of ripped CD's etc) and got this message:

"To listen to your songs and playlists across all your devices, music on this computer’s library will merge with Cloud Music Library."

If I say yes, what happens? Will it try to load my 160gb+ music library onto my iCloud and then tell me I don't have space? ....orrr what?

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