As outlined in the Conversion Pixel Variables doc, the Transaction ID parameter in the pixel is your Advertiser's opportunity to pass a unique value associated to the conversion to cupCAKE so that you can audit your conversions against theirs.

So if you recorded 10 conversions, and your Advertiser recorded 8 - you could pinpoint which 2 conversions account for the discrepancy based on matching up our transaction ID's with your Advertiser's reports.

Where does the transaction ID show up in cupCAKE?
If you received this pixel from your Advertiser:

That value displays in the conversion report under the "Transaction ID" column

The conversion card would register that value as the Transaction ID

And this transaction ID is searchable in the main search bar under "conversions".

You can enable a setting to dedupe conversions with the same Transaction ID by enabling an Offer-level setting. 
Location: Offer card > Home tab > Attribution sub-tab

Enable Transaction ID Deduplication - enabling this setting will reject any conversion pixels with transaction ID's that have previously been received.
Once checked, a 2nd checkbox will appear.
Transaction ID Deduplication by Advertiser - will dedupe across any Offers under that Advertiser that have the previous checkbox checked. If only 1 Offer has that checkbox checked, this second setting will not work.