Each document in an index is assigned a unique integer identifier called a "DocId" or document id. The first document added to an index has the DocId 1, and subsequent documents will have sequentially numbered DocIds 2, 3, 4, and so forth. When a document is reindexed, its DocId is "cancelled" and a new DocId is assigned. Compressing an index renumbers all of the DocIds, so after an index has been compressed, a document's DocId may change.
DocIds are used in:
SearchFilter objects, which use DocIds to identify the documents selected in the filter.
SearchResults.AddDoc, which lets you add a document to a SearchResults list by providing the index path and the DocId. You can use this to get the properties of the document from its DocId.
The IndexJob.ToRemoveListName list, which lets you remove documents from an index either by name or by DocId.