The site the request acts on, e.g. s2. You find it in the remberg web app URL (e.g. /s2/) or next to the site name under Settings → Sites. If your account has access to more than one site, send this header on every request; site-specific requests without it fail with 403 ACTIVE_SITE_UNAVAILABLE. With access to a single site, that site is used when the header is omitted. A site your account cannot access fails with 403 ACTIVE_SITE_UNAVAILABLE; a record that belongs to another site returns 404.