The marker pictured is the sliding stitch marker. As you can see, it is just like a chart stitch marker, but it has a control to slide it back and forth easily from the main page. There is also a sliding row marker that looks just the same but runs the other way. These markers are available for both text and chart pieces. You can control the width and color of the marker, as well as the location of sliding control. You can easily hide and show the slide control by a single tap that toggles it on and off on the main screen (tapping to make the slide control disappear will not affect the visibility of the marker itself). You can turn the control off entirely as well. All of these settings are available under Preferences.
I find the sliding stitch marker so very useful to keep my place on a row that is wide and complex. Especially on the phone, or even the iPad when the chart I’m using is wider than my screen at a comfortable viewing zoom, this sliding marker makes it super easy to keep my spot as I scroll side to side to knit a row. On written instructions, especially for lace, I use it to help me track where I am on a row, by using the junction of the row marker and the sliding marker. The sliding row marker is very useful for those shorter pieces that you don’t want to setup a text row marker for.