In having the clan of Carrión cast doubt on the noble pedigree of Rodrigo of Vivar, the poet has chosen to give a particular emphasis to the final part of his tale, the section that a listening public or a reader of the manuscript would experience last and go away remembering. Since, as most commentators would now agree, the relationship between the Infantes de Carrión and the Cid's daughters is largely and perhaps entirely fictitious, the afrenta de Corpes, the court scene at Toledo, and the three trials by combat – that is to say almost the entire second half of the poem – must also be fictitious. The confection of those episodes is a key to the poet's purposes. He does what other epic poets composing in the Romance traditions did before him, appropriating the name and some historical details from the life of a figure who has caught the imagination of his compatriots and employing them in his narrative for his own purposes (see Duggan 1985). Those purposes are revealed by the selections that the poet makes in telling his tale.

The primary choice is, of course, that of the principal topic, the rise to power of Rodrigo Díaz of Vivar and his relationship with Alfonso VI.

A second area of choice is found in discrepancies between the poem and what is known about the Cid and his contemporaries from the historical record.