How the budget basket works
Positions are ranked by profit efficiency (profit per ISK invested) and filled greedily until the budget or EVE's 150-order limit is reached. Each position is capped at N days (your risk profile) of the item's average daily volume — buying more than the market trades in that window means you'd be stuck holding stock nobody wants to buy from you yet.
Want the full explanation — profit efficiency, days-of-supply sizing, and a worked example? Read the EVE Online trading budget guide.