Which feature of the REST API would you use to upload documents into a search index?

The correct feature to use for uploading documents into a search index in the context of the REST API is the index. An index is essentially a data structure that allows the search engine to efficiently store and retrieve information. When you upload documents directly to an index, those documents become searchable and are organized within the index according to the schema that has been defined.

In Azure Cognitive Search, the index serves as the primary storage mechanism for the searchable content, holding the documents and their associated fields. By directly interacting with the index through the REST API, you ensure that the documents are appropriately indexed and can be queried effectively by users.

The indexer, on the other hand, is a separate entity that automates the process of pulling documents from a data source and populating the index with that content. While the indexer is an important feature for automating data ingestion, it is not used for direct uploads.

Skillsets are used to enrich the content as it is indexed, employing various cognitive services to enhance data before it is searchable, while analyzers are concerned with examining the content of the documents and breaking it down into manageable search tokens. Neither of these features is intended for the direct upload of documents.