Many Garmin outdoor units (including the 66i) does implement the user grid and user datum settings. Traditionally, Garmin is laconic about these features. Basically it just says that it is only for an advanced user and that "This option should only be used when the other position format options available on the Garmin do not match what is on the paper map". Indeed this is by no mean a fitness-related feature and a relevant background from the user's side is required. Moreover because of the implementation-specific moments, it can provide considerable guesswork to any user.
Sometimes, it's not easy to guess. For instance, the spheroid parameters data fields are apparently too short but it's easy to realize that Garmin uses the Delta system where the value is expected as a little offset from certain known base, so the field length is just enough. By walking through manuals for other Garmin products, it can be noticed that the delta is defined as (WGS84 - Current), and not vice versa. That's OK. But for the flattening difference, the value is so small the first significant digit is located far right outside the input form making the value impossible to input. Fortunately, examples on the user's forums suggests that the value is expected to be multiplied by 10**n to get rid of the leading zeroes. Too hard to guess without such hints.
This is for user datum. As for user grid, the firmware provides several templates allowing to define a grid using different projections (UTM, Lambert, stereograph, etc.). Out of curiosity, I've tried to define the same grid based on Transverse Mercator projection using two different templates, UTM and Oblique Mercator Point & Azimuth. My assumption was the Transverse (UTM) is a particular form of the Oblique projection with the tilt angle (azimuth) of 90 degrees. So the coordinates are expected to be the same provided the grid parameters in each template are set accordingly. But it turned out this is not so simple because the coordinate readings were totally different. I've no idea why but I've noticed that typically, the units are displayed in the input form page, but not for the azimuth one. So it's not clear of what units the azimuth value is expected (inches, townships, Fahrenheits or even more exotic units).
By the way, it's stated in the release notes for an old firmware that "Fixed issue with Oblique Mercator User grid". A fix implies there was a defect. A defect is usually defined as a difference between expected behavior and an actual one. If an expected behavior is not specified, then by definition, there can be no defect. So the statement is confusing a bit. Anyway Garmin is presented on the internet very extensively. It has many web sites each of which is huge. So the chances are this is a typical case of overlooking. Perhaps a document with comprehensive coverage of the topic was created long time ago by the Garmin's SW department, reviewed in the literature department and approved by the vendor for public release. It's just my failure of not finding it. So it would be great if anyone can provide a link.