cursorMark is stateless and how it solves deep pag

as specified here cursormark is stateless but I don't get how it it is solving deep paging issue if its stateless. Does solr stores the indexed data in sort by unique key field if so then it will clarify my confusion.

if I am wrong please explain how the cursormark solves deep paging. Because as cursormark is stateless it also need to sort and claculate cursormark every time request a query and this is similar to start=#start-position.

From the link you referenced...

Cursors in Solr are a logical concept, that doesn't involve caching any state information on the server. Instead the sort values of the last document returned to the client are used to compute a "mark" representing a logical point in the ordered space of sort values. That "mark" can be specified in the parameters of subsequent requests to tell Solr where to continue.

This is elaborated further in an explanation of the constraints on using cursorMark...

Cursor mark values are computed based on the sort values of each document in the result, which means multiple documents with identical sort values will produce identical Cursor mark values if one of them is the last document on a page of results. In that situation, the subsequent request using that cursorMark would not know which of the documents with the identical mark values should be skipped. Requiring that the uniqueKey field be used as a clause in the sort criteria guarantees that a deterministic ordering will be returned, and that every cursorMark value will identify a unique point in the sequence of documents.

If this doesn't help clarify things for you, the next best explanation i can offer is to think of the cursorMark as an encoded filter telling Solr to skip all documents with values in the sort field(s) that come "before" (based on the asc/desc sort order) some specific values.