Often a general needs to write on the map, better to direct his forces as they route in the face of the enemy, or charge positions. And Map Marks are provided for just this.
There are several ways to create a map mark. One is to use the menu option to open the map marks dialog, you can then type in the coordinates and the label that you want. Click ADD to add the new mark to the list, and then OK to close the window. The map mark will now be drawn on the map and will also be added to the map marks menu.
Alternatively, you can move the mouse onto the hex you want to mark and then press 'b'. This will again open the map mark window, you should then type into the name field and then click on add, and finally OK. Again the map mark is added to the map and the menu.
Selecting map marks from the menu will let you jump directly to that part of the map.
There is no automatic way to share map marks with your allies. One way to share a big map mark list is to use a text editor to open the game_client.properties file in the Taratus install directory. This lists all map marks and you can copy this and send to allies in emails. They should then copy the text into their own game_client.properties file.
If you do this then ensure there is only a single mapmarks entry in the properties file.