US Standard Coffee Cups are 5 Fl Oz or 147ml. Different from the US cooking "Cup" which is 8 fl oz or 236ml. UK mugs are typically 250ml.
The water reservoir is marked in cups and ml but with 150ml markings. Luckily the system can be told to brew quantities in 10ml increments so 500ml for 2 mugs is easy to set as a default. This all causes much confusion.
The user guide is the common 5 pages of pictures. So you really need the online FAQ. Which is hidden away in the marketing website.