Does the field used in field_value_factor function scoring need to be an 'indexed' field, or can it just be 'stored'?

I need to use a positive-valued float or double value field, one field value per document, that I compute at index time. I want to use this value to boost the score of each document matching an arbitrary query, by the value of this field. I plan to use a function score query, with field value factor as the function, for this purpose.

The field will never be used directly as a term in a search query (term query, match query, etc.). It's sole purpose is to be used for multiplicative boosting of the scores of documents that match other queries.

My question is, should this field be indexed (in this case, what should be the mapping and the type of analyzer), or can it be just a 'stored' field?

Would appreciate any clarification on this.
Thanks a lot!

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