A pack size and a minimum order shape how much gets ordered, not when. Reorder points and max levels decide when - see Set reorder points and stock levels.
Set them on the product
In the product page's Supplier ordering section you can set, per variant: Supplier SKU (their part number for the item), Pack size (how many units the supplier ships together), Minimum order, and a Pack name ("case", "box" - one short word of up to 12 characters; a longer name saved earlier reads as "pack" in the order's hints). These shape quantities rather than thresholds: reorder suggestions lift to the minimum and then round up to whole packs, and the PO editor shows the breakdown - "3 cases of 4" - with a caution on quantities that break the pack or sit under the minimum. Quantities everywhere stay in single units - packs are context, never a forced rounding. A new line on a purchase order starts at the case quantity when the item has one (or the minimum, rounded up to whole packs), so a product that ships six to a box lands as 6 - and you can type any number over it.
Buy in bulk, sell by weight or volume
A refill shop buys dish soap in 20 L jugs and sells it by the 100 g. Nothing is bottled or mixed, so this is a pack size, not a build: set Pack size on the product you sell to the number of 100 g units in one jug, and Pack name to "jug". A purchase order then reads "3 jugs of 206", the cost can be typed per jug on the order line, and receiving adds 618 units to the product you sell.
Press Work it out under Pack size to do the arithmetic. Enter Bought as (20 L) and Sold per (100 g). A Density box appears when one is a volume and the other a weight - it is on the supplier's spec sheet, or weigh one litre. The panel shows the answer and the sum, and always rounds down: a 9.75 L bag at 1.03 g per ml gives 100 units of 100 g, not 101. Use fills in the pack size, and the sum stays under the row until you type a different number by hand.
Six scents in the same jug? Set for all variants, above the rows, applies a pack size, pack name and minimum order to every variant in one go, and has its own Work it out. Jugs vary a little and product gets spilled, so run a stocktake now and then to true the stock up.
Set them from a purchase order
You can also set all three without leaving a purchase order: in the PO editor, click the pack note under a line's quantity (or Set case size on a line that has none) and enter the pack size, pack name and minimum order right there. It saves to that PO's supplier for the product, so future orders and reorder suggestions use it too.
More than one supplier
Buy the item from more than one distributor? Each of them can carry their own part number, case size and minimum in the same section, under Other suppliers for this item - see Buy the same item from more than one supplier. The values at the top of the section are the fallback, used for any supplier without a row of their own.