In the Sogetsu school of ikebana (we only talk about the Sogetsu school because that is what we are intimate with), the ikebana practitioner is allowed to have her own kanban after she obtains the Teacher's Diploma Fourth Grade. The diploma also means she can now start teaching ikebana. In the old days, many people taught ikebana from their homes. They hung their kanban outside to let people know ikebana was taught in that house. It is similar to the Western practice of a doctor or lawyer hanging out their shingle when they begin their practice. In this age of the Internet search engines, the kanban has probably outlived its usefulness but we imagine most teachers would still want to have one...if only for sentimental reasons! The kanban is not compulsory and you do not need it to start teaching. You can teach as long as you have the diploma and you are registered as a "teaching" member of the Sogetsu Teachers' Association! (Note: You can also register as a "non-teaching" member but this means you cannot take on students.)
Let's take a closer look at the kanban. The big white characters「草月」say "Sogetsu". The square symbol near the top left is the official seal of the Sogetsu school. The smaller characters in black on the middle right「師範」say "shihan" which means "teacher" or "instructor". The characters in light blue on the bottom half「バレステロス虹都」are Miyako's name...or rather her gagoh (雅号). It says "Ballesteros Kouto".
Gagoh can be loosely translated as a "pseudonym". Many people practicing the arts (ikebana, shodo, chado etc) in Japan use a gagoh when they reach a certain level of mastery. In the Sogetsu school, students who have earned the Fourth Certificate* (１級修業証) can start using a gagoh.
*Note: The Fourth Certificate is obtained after successfully completing Curriculum 4. The Fourth Certificate is not yet a teaching diploma. See here for more information about the Sogetsu ranks.
In Sogetsu, the gagoh is a name you can choose for yourself; but more often, it is your teacher who gives the name to you. In Japan, the study of ikebana is a lifelong endeavour for most people. Once a teacher has been chosen, students study under that teacher for the rest of their lives, except when their is a special reason to change. The gagoh is normally given in kanji, where one of the characters is taken from the teacher’s own gagoh. Miyako's teacher’s gagoh is “shikou” (紫虹 “purple rainbow”). Miyako's gagoh is “kouto” (虹都 “rainbow capital”). Incidentally, the character "to" (都) can also be read as "miyako" in Japanese!
Again, the gagoh is not compulsory but most people like to have one. The gagoh gives recognition of accomplishment and also a sense of continuity from teacher to student. If you do elect to have a gagoh, this must be duly registered with the Sogetsu Foundation. The gagoh appears in official documents like the Sogetsu Teachers' Association Membership Card.
It also appears in your diplomas.
Did you know....?
The gagoh of the founder of the Sogetsu school is "Sofu" (蒼風). Therefore, in Sogetsu no one is allowed to use the character「蒼」"so" in their gagoh. Likewise, the use of characters that will be pronounced as "Sofu" is also prohibited.
Sources:
草月流非公式用語辞典
花の情報局
How to Study Sogetsu Ikebana