A "complication" is an area on a watch face with additional information. This is as old as watch faces, but it's digital now.


It is helpful to understand how Apple Watch complications work. An issue with these is that third-party complications are limited to 50 updates a day and a maximum of 4 per hour. This update budget is shared among the third-party complications you have selected to be shown on the watch face. It is intended to ensure your watch lasts more than a few hours on a charge. CGM updates 288 times a day, 12 times an hour. These don't line up well when you want current data.


Because of this restriction, you will often get old or no data from a complication when you view your watch face. Sugarmate, Loop Follow, xDrip4iO5, and other apps use the Apple Calendar complication to work around this. The apple provided complication for the calendar is not limited. This approach provides real-time values.

Note that this may increase battery consumption and reduce the time between charges.
You can see here that the bottom middle (The Loop app watch complication) has an open red loop and "--" for data; however, the calendar complication has current detailed information, including the glucose value and closed green Loop status. The complication on the bottom left coincidentally has been given a piece of the third-party update budget and has a current value.


Using those third-party complications will run the related watch app when pressed, which will immediately update current data and allow other operations. For instance, the Loop app complication allows me to enter carbs, bolus, and enact temporary overrides. All from the watch face.


Hope this helps