In the Sample Style.qml file there is a symbol category for “Post Hoc Write-Off.” This symbol, a green dot with a grey X over it, is for marking samples that upon reflection (such as comparison to aerial or satellite imagery) should have been written off in the field, but were initially tagged as Positive samples where collections were made. To display this symbol, change the Status attribute of the sample feature from “Positive” to “Post Hoc Write-Off,” and QGIS will update its display automatically. Note, however, that if you do this you should also set all of that feature’s measurements to NULL, as they would have been if the target location was written off in the field.