How do I properly tag products for new collections in my online store?

[RoboCode2000]

Question...  New here, New Store... trying to understand how this works. This is what I observed first time on our site build. Please advise if anything here is not correct. 

CSV File (Inventory) - the spreadsheet begins with these headers in the top column: Handle, Title, Body (HTML), Vendor, Type, Collection, and a lot of other data, etc.
This file is about 15,000 items deep, sorted by Collection.
I made a copy of that file for analysis, and with a couple of formulas, I was able to drill down to 50 collections, of common like Products.

CSV file is Uploaded and populates PRODUCTS. Each item is listed, with relevant data for each item. 
PRODUCTS populates COLLECTIONS, where similar products are separated by Collection (as listed from the CSV File). Right away I have 50 COLLECTIONS, identical to CVS FIle.

I need to create a few new Collections, where I want to bring together "Best Selling" or "Womens Clothing" & "Womens Shoes" or whatever query I need to create the Collection I need.
The problem I am having is this: When I create the new Collection (Type - Automated)  - Conditions, there is nothing in the first drop down list that will allow me to relate a Product Tag, or Type, to any given Product. The "Key" field for the TAG I expected to see would be Collection, but it was not found.

How does it know where to put each item in the correct Collection without an obvious TAG? 
Having to create a new Collection, I have to find a way to Identify how each Product is "Tagged".
Having to sort through 15000 items to add TAGS for the new Collection would be a nightmare.

What am I not seeing? Hope this illustration makes sense, lol
Please advise and in advance, Thank you.

[productsync]

Hi

The easiest way is when you import your products you use "Tags" column to add all the collections you want the product to be added to: https://help.shopify.com/en/manual/products/import-export/using-csv#product-csv-file-format

So when you create an automated collection select Product tag to use and replace "Collection" column of your csv file to "Tags" during import. The first column of setting up an automated collection is set and not dynamic so it will never display Collection.  https://help.shopify.com/en/manual/products/collections/automated-collections/auto-select#add-select...

Hope that makes sense.

Regards

Tom

[RoboCode2000]

Thanks for the response.
Actually, after some research, the information on the help pages call the Key field "TYPE"

https://help.shopify.com/en/manual/products/import-export/using-csv

Our file had an extra column "Collections", in between "Type" and "Tag"

So this is now understood, thanks again.