Creation Baskets and Custom Baskets

Every trading day, an ETF issuer publishes a creation basket — a precise list of securities, and the quantity of each, that an authorised participant must hand over to receive one new block of ETF shares (a "creation unit," often 50,000 shares). Redeeming works the same way in reverse: the AP hands back a creation unit and receives the basket. In the plain-vanilla case, this basket is simply a slice of the fund's actual portfolio, scaled down to the creation-unit size, so creations and redemptions barely disturb what the fund holds.

Funds don't always use that plain slice, though. A custom basket is a modified version — perhaps light on a hard-to-source or illiquid bond, or including a cash component to cover a security the AP can't easily deliver that day. Regulators require issuers to have a written policy for when and how custom baskets get used, precisely because a basket that's quietly tilted can favor one AP over another or let the issuer offload unwanted holdings onto redeeming APs.

The basket mechanism is what lets an ETF track an index of thousands of illiquid municipal bonds, or a foreign-market index that's hard for retail-sized trades to access at all: the AP, not the fund's portfolio manager, does the work of sourcing the underlying securities, and the fund simply exchanges shares for whatever basket its policy specifies that day.

For an equity ETF tracking a liquid, large-cap index, the creation basket is usually about as boring as it sounds: a pro-rata slice of the index, published every morning before trading opens, changing only when the index itself rebalances. It's in fixed income and international equity ETFs where custom baskets earn their keep — a bond fund might substitute a similar-duration, similar-credit-quality bond for one that hasn't traded in weeks, or accept a cash payment in lieu of a bond that's simply unavailable that day, so the AP isn't forced to hunt down a security nobody is selling.

The basket composition is published daily and is one of the most closely watched pieces of information around an ETF, since it's effectively a real-time window into exactly what the fund holds — a level of transparency plain mutual funds, which disclose holdings only monthly or quarterly, don't offer.