Both Comiket and COMITIA are doujinshi conventions held in Japan. Both conventions are generally the same event with several differences. See What events usually happen at Comiket?
While both Comiket's and COMITIA's main focus is socializing and exchange doujinshi. COMITIA is the largest dojinshi convention focused completely for the exchange of original dojinshi in all genres. So unlike in Comiket, any doujinshi based on an existing work are not allowed in this convention. And unlike in Comiket where there are large numbers of cosplayers participate, there is no cosplay at COMITIA.
Frequency and Location
Comiket is held twice a year in Tokyo, in August and December. Generally referred as NatsuComi (Summer Comiket) and FuyuComi (Winter Comiket), in Tokyo Big Sight convention center. While COMITIA main convention is held four times per year located in various convention sites in Tokyo depending on the size of the edition in question. And associated COMITIA cons are held in other parts of Japan as well. There are smaller editions are held throughout the year, 2 times per year in Osaka, Nagoya, and Niigata.
Comiket is way larger than COMITIA. In Comiket, the number of circles that can participate is limited to 35,000 where there are more than 30.000 circles participants in recent years. Where in COMITIA, about 2000 to 3000 circles have participated in every COMITIA in recent years. The hundredth edition of the main con, is set to welcome about 5000 circles.
Some professional mangaka also participate in COMITIA, selling their pro works directly to fans. But they must participate as individuals; company booths are not allowed. While in Comiket there are more than 100 companies, ranging from manga publishers to anime production houses to dojinshi resale shops and other fan-oriented companies, like Pixiv, can participated in Comiket. These company booths are located on a separate floor and have a separate catalog. Companies at Comiket generate publicity for existing and upcoming projects, and sell goods, often limited editions that are distributed exclusively at Comiket.
The first Comiket was held on 21 December 1975. While the first COMITIA was held later on 18 November 1984.
Similar to Comiket, the number is the identification of the n-th edition of the COMITIA. Currently they have COMITIA122. The hundredth edition of COMITIA took place on 5 May 2012.