What kind of drawing format is this? Pictures with litle box on top left
What kind of drawing format is this?


It's a "Circle Cut".

In this context, "Circle" means "Doujinshi circle", and "Cut" means "Cut-out". It's exclusively used in a doujinshi market place such as Comiket, Comitia, etc to be used in the catalog.

The format is:


A little box on the upper-left: Indexing/Stall location (to be explained later)
Text inside the box on the upper-right (or none): Circle's name
The rest of the area: Free image, can be a unique image for identifying the artist, or anything related to the circle's current merchandise



  What's the little box on top left for?


The little box on the top-left is used for Indexing/Stall location. Inside the catalog, there's a list of circle-cuts with their stall location, as shown for example in the Comiket 90 catalog:



Catalog for Comiket 90, date 12 (Friday), East Hall 4-6. Left page showing stall モ43-モ60, right page showing stall ヤ1-ヤ17

For reference, this is the map of Tokyo Big Sight for East Hall 4-6:



Map of Tokyo Big Sight, East Hall 4-6


  And what's the meaning of 'C..' on title? And what does the number stand for?


It's the identification of the n-th Comiket.

As mentioned before, it's used in a doujinshi market like Comiket. Comiket is done 2 times per year (summer &amp; winter) and each event is always identified by its serialization/number, such as Comiket 92 (C92) in this Summer 2017.



Reference:


What the heck is a "Circle Cut"?!
Comiket Event Report Part 2 - Guidelines for Comiket