"Carryover" here refers to the etymological meaning of metaphor (ultimately from Greek μεταφέρω (metapherō), "to carry over", "to transfer"). In a comparison, attributes or qualities of the figurative term (a mare among Pharoah's chariots) are transferred or carried over to the subject (the woman). These terms, the thing being compared (woman) and the thing to which it is compared (mare), are usually referred to as tenor or ground and the vehicle or figure respectively.

Carryover (noun) refers in this context to that which is carried over or transferred: that is, the meaning ascribed to the tenor/ground through comparison with the vehicle/figure. The carryover here is value: the value of the mare is carried over to the woman. But carryover in metaphorical comparisons will usually include a whole range of additional connotations and attributes: the mare is not just valuable, but also regal, strong and beautiful, among other things. The richer and more applicable these connotations are, the more expressive the comparison.

--------------------------------------------------
Note added at 2 hrs (2016-09-27 11:14:15 GMT)
--------------------------------------------------

An example or two to illustrate the meaning of "carryover" in relation to metaphorical comparison:

"Here again in either the metaphor or its converse there is a carryover from the feeling of the literal sense to that of the figurative."
https://books.google.es/books?id=Y6TzgsS035kC&pg=PA99&lpg=PA...

"For adults, the criterion for aptness in metaphor appears to be maximal carryover of relations with minimal carryover of attributes; in contrast, we theorize that the aptness criterion for literal similarity is simply maximal carryover of predicates of all types (Gentner, 1983; Tversky, 1977)."
https://www.ideals.illinois.edu/bitstream/handle/2142/17632/...