Buffalo Niagara International Airport (KBUF) does not have an associated VOR or VORTAC, yet it has a published frequency box that lists a VHF frequency (116.4) in parentheses. Why is this frequency in parentheses and what does it designate? See below:
Complementing William's answer; my go-to for such intricacies is the specifications used to create the charts by the Interagency Air Committee. Which here also serves as a formal source that includes the word "parentheses":
DME facilities shall be shown as illustrated in Appendix 6.
DMEs shall be identified by name, channel, identification letters, Morse code, and paired frequency in parentheses.
And from said Appendix 6:
As you can see, the whole DME "box" is a perfect match.
There is a VOR/DME for BUF on that frequency, BUT it's either been, or is in the process of being partially decommissioned, leaving just the DME portion.
And when you've got just a DME you put the frequency in parenthesis.