[TRACK_ID] is OPTIONAL parameter which could be replaced with some helpful info (usually transaction ID) on the advertiser side. That parameter will be assigned to EFFECTUS transaction and could be found at the Reports - Transactions section. Column "Track ID"

Example

Pixel code: <iframe src="https://NAME.oneffect.us/affiliate/pixel/5/track1/" height="1" width="1" frameborder="0" scrolling="no" /></iframe>