The generic mkfs command calls a file system-specific mkfs, which then creates a file system of a specified type on a specified disk slice. Although mkfs can support different types of file systems, in practice you would use it to create UFS or PCFS file systems. To make other types of file systems, you would have to write the software for the file system-specific versions of the mkfs command to use. Normally, you do not run mkfs directly; it is called by the newfs command.
The generic mkfs command is located in /usr/sbin. See mkfs(1M) for a description of the arguments and options.