Cart quantity
The total number of items in the cart. Two of one product plus one of another is 3.
Example: Cart quantity is at least 3 gives free shipping on 3 or more items.
Every item counts, including gift cards, digital products and free Buy X Get Y items.
To count only certain products, use Item count from collection. See Collection conditions.
Cart weight
The total weight of the items in the cart, using each product's weight in Shopify.
Example: Cart weight is at most 2 kg discounts shipping on light parcels.
Pick the unit before you enter the number. It defaults to g (grams). Choose kg, oz or lb if that's what you mean. "20" in grams is not "20" in kilos.
Products are converted automatically, so products weighed in different units are fine.
Products with no weight set count as 0.
Operators
Is at least, Is greater than, Is at most, Is less than, Is equal to.