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Screening ETFs by asset class uses the four-level holdings taxonomy as the filter vocabulary: asset_class at level 1, category at level 2, segment at level 3 (the peer-group key), and specializations at level 4. Because the labels are computed from holdings rather than fund names, a screen on these codes returns funds by what they actually hold.
Plan: Fund · Credits: 1 per call; screen facets and the taxonomy endpoint are free

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The Asset-Class Filters on the ETF Screen

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Comma-separated level-1 codes; matches ANY. The values are equity, fixed_income, multi_asset, commodity, crypto, currency, volatility.
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Comma-separated level-2 codes, e.g. equity.sector, equity.size_style, equity.thematic, fixed_income.corporate, fixed_income.municipal, multi_asset.allocation, crypto.futures. There are 38 categories; the free taxonomy endpoint lists them all.
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Comma-separated level-3 codes — the peer-group key, e.g. equity.sector.health_care, fixed_income.corporate.high_yield, equity.size_style.emerging, equity.leveraged_inverse.single_stock.
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Comma-separated level-4 codes; matches funds carrying any of them — the only multi-valued level, e.g. equity.sector.health_care.biotechnology, equity.thematic.ai_robotics.
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A dotted-path prefix matching any level: equity.sector is every sector fund, fixed_income is every bond fund, equity.leveraged_inverse is every leveraged equity fund.

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Codes Are Exact — an Empty Page Means a Misspelling

The taxonomy codes cannot be guessed, and a misspelled code returns an empty page rather than an error — which reads exactly like “no such funds exist”. Retrieve the vocabulary from the free taxonomy endpoint, or call the free screen facets endpoint, which returns the codes as actually carried by funds with a count for each.

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Labels Answer “What Is It”, Not “How Big Is the Position”

A taxonomy filter selects funds whose book concentrates in that node under the stated per-node rules. To bound the actual measured share of something — US weight, large-cap weight, treasury weight — combine the label filter with the measured filters documented on Screen ETFs by Portfolio Shape and by Geography. “Large-cap biotech” is a segment plus a specialization plus a measured floor, because sector funds carry no cap-band label. By geography · By wrapper and structure · By size and fees · By portfolio shape · By flows and premium · ETF screening overview