The ampersand (&) joins the expression preceding it with the one following it. With:
"'[2015 Cordell Valuation Lookup Tables.xlsx]"&BK2&"'!$A:$I"
there are thus three expressions being joined:
• "'[2015 Cordell Valuation Lookup Tables.xlsx]"
• BK2
• "'!$A:$I"
The first and last of these are literal strings; the second one retrieves the contents of BK2. So, if BK2 contains 'Data type 1', you end up with a VLOOKUP formula referencing:
"'[2015 Cordell Valuation Lookup Tables.xlsx]Data type 1'!$A:$I"

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]