In short: Cart Subtotal and Cart Total check the value of everything in the cart, and Cart Quantity counts every item in the cart. Use them for rules like "spend $100 or more" or "buy 3 or more items".
The three conditions
Cart Subtotal: the value of all items in the cart at their normal prices, before any discounts.
Cart Total: the value of all items after any price reductions Shopify has already made to them, but before this discount. Often it's the same as Cart Subtotal.
Cart Quantity: the number of items in the cart. Two of the same shirt count as 2.
None of them include shipping. If your prices include tax, the values include tax too.
Operators
Money (Cart Subtotal, Cart Total): Greater than or equal to, Greater than, Less than or equal to, Less than.
Quantity (Cart Quantity): the same four, plus Equals.
Worked examples
Spend $100, get 15% off: Global Condition Cart Subtotal, Greater than or equal to,
100.Buy 3 or more items: Per-Discount Condition Cart Quantity, Greater than or equal to,
3.Small orders only: Cart Subtotal, Less than,
50.
Cart conditions vs Group conditions
Cart conditions look at the whole cart, including items the discount doesn't target and items you've excluded. Product discounts also have Group conditions that only look at the items that discount targets. See Group and Included Items conditions.
Example: 10% off the Summer Collection when the customer buys 3 items.
Cart Quantity at least 3: any 3 items in the cart count, from any collection.
Group Quantity at least 3: 3 items from the Summer Collection are needed.
Good to know
Thresholds don't subtract the discount this code gives. For example, "Cart Subtotal at least $300" is met by a $310 cart even if a 20% discount from this code brings it below $300. For "spend $X" rules, Cart Subtotal is the safest choice because it never changes with discounts.
Values are in your store's currency. Customers paying in another currency are converted automatically. See Using OneCode with multiple currencies.
Cart Quantity isn't available as a Per-Discount Condition on order discounts. Add it to Global Conditions instead.