The left click/tap is for enumeration with pre-focus. the right-click is for reverse enumeration. In each cell, you can make notes, inserting up to 20 digits and other characters.
When clicked, green areas are highlighted to show the spots which are affected by the selected cell. The cells highlighted red contain inconsistencies with Sudoku rules. Use "/" (Slash) to put all possible variants in a free cell.
To place or erase a number in a cell, use the left mouse click. To erase everything and put the selecetd number into the cell, right-click the mouse.