Tog is the measure of thermal resistance that indicates  the warmth to weight ratio of a duvet - the insulation value.

 If  you sometimes feel too warm in summer, and  too cold in winter, then one solution is to have two separate duvets, one for each season. But an ALL SEASONS duvet has two duvets clipped  together  a 4.5 tog and 9  tog which means you can use either as appropriate, or clip together for the equivalent of  13.5 tog. To summarise, by season:

Summer: 3/ 4.5 or 6 tog

Spring/Autumn: 9 or 10.5 tog

Winter: 12 or 13.5 tog

All Seasons: 4.5 + 9  = 13.5 tog