Seminar falls in Class 41, education & entertainment, and depending on what exactly you sell it can also fall in Class 16. Everything below is read from the USPTO's own list of accepted descriptions, not from an opinion about where it belongs.
Searching the USPTO Trademark ID Manual for seminar returns 55 accepted descriptions, and most of them sit in Class 41. The manual is the list of identifications the office has already approved, so wording taken from it is pre-approved and settles the class at the same time.
The class follows from the identification you write, so "what class am I in" is really "what exactly do you sell". How the class system works covers the rest.
You teach, train, publish, coach, or provide entertainment, events, or media content.
54 accepted descriptions in the manual mention seminar. For example:
Paper products, printed materials, books, or office and art supplies.
1 accepted description in the manual mention seminar. For example:
Seminar appears in 2 classes because what you make, what it is made of and the service of selling it are separate classes. Pick the descriptions above that match what you actually do.
Seminar is a service, so it sits in Classes 35 to 45. Software that delivers a service is often Class 9 or Class 42 as well.
Education and entertainment: music education and personal fitness training are here. Recordings are goods, so downloadable podcasts and downloadable music are both Class 9.
The class decides what a registration covers; it does not decide whether the name is available. Relatedness is judged on the identifications, so a conflict can sit in a class you never looked at: the same name in a different industry.
Check whether your name is clear for seminar Searches the live USPTO register, including sound-alike and meaning-alike names. First search $2, then $5.Seminar sits in Class 41, education & entertainment. That is where the USPTO's own ID Manual puts it: 54 accepted descriptions mentioning seminar fall in Class 41. Depending on exactly what you sell it can also fall in Class 16.
You file in every class you actually use the mark in, and the USPTO charges its fee per class. Selling a product and also running an online store for it is two classes, not one. Filing in classes you do not use is worse than filing in too few: an application has to be supported by real use or a genuine intent to use.
No. The class decides what a registration covers. Availability turns on whether someone already holds a similar mark for related goods, and relatedness is judged on what the identifications say rather than on the class number, so marks in different classes can still conflict.
Not what you sell? Search the class finder for your own wording, or browse all 45 classes.