When you visit a museum and the cat paintings are grouped together in one room, and the fruit paintings in another, you could conclude that the curator favors a thematic arrangement, meaning grouped by topic, rather than chronologically or by artist.
Thematic relationships are everywhere. You could apply a thematic arrangement to your closet, putting the 1970s disco-wear in one section and your motorcycle gear in another. When you're studying the works of Charles Dickens, finding the thematic similarities across all the novels helps you remember them more easily. That song your garage band has been working on has a thematic development, too, starting with a line of melody, changing it a little, taking it in a different direction, then returning to the theme.
It is said that The Grohmann Museum, at Milwaukee School of Engineering, is home to the world's most comprehensive art collection dedicated to the evolution of human work. Have you visited the museum lately? Can you identify some of the sub-themes being exhibited?