Every record has some meta fields that are providing some meta information on the records.
For example you can get the creation date, the status, etc. All these fields are prefixed with an underscore, let's see them in detail:
_createdAt: date of creation of the record;
_firstPublishedAt: date of first publication of the record;
_isValid: is the record valid? This can be false if the schema has changed and the records haven't been updated yet;
_modelApiKey: the API key of the model;
_publicationScheduledAt: if the publication of a record is scheduled in the future, this field will hold the publication date;
_seoMetaTags: it's an object with the SEO meta tags computed from an optional SEO field and the fallback details from the main site settings. It's an object representing the meta tags:
attributes: the meta tag attributes;
content: the meta tag content;
tag: the meta tag name;
_status: represent the record status: draft/published;
_updatedAt: it's the date of last update;
All these fields are read-only (also use the CMA) as they either represent an internal state of the record or they are precomputed by our API using other records (e.g. SEO fields).