Receipts generated for gifts outside of RE (like from 3rd party vendors accepting donations on behalf of your organization) cannot yet be imported to the Receipt Number field on a gift, as the field only accepts numbers. However, many 3rd party vendors issue receipts with alpha-numeric values. Organizations in Canada will have seen this with Canada Helps-issued receipts, but there are many other vendors who do this (presumably an alphabetic digit ensures no duplication with an org's CRM, which usually generates tax receipts as numerical only).

Right now, the workaround is bringing in the receipt as a Gift Attribute, which is fine, but not consistent if you're also issuing receipts via RE. Then you have receipts living in two separate fields.

For an example of a non-profit CRM with this ability, see Salsa.