TOPS Terminal imitates several different types of terminals. The type of a terminal determines how it interprets special "escape sequences" which may be received from another computer. For instance, there are escape sequences to clear the terminal screen, to move the insertion point to a particular place on the terminal screen, to clear the current line from the insertion point to the right margin, and so forth. Escape sequences are used by service computers to provide such services as editor programs, user-friendly bulletin boards, graph drawing programs, simulated board games, and so forth.
The most universal terminal type is the DEC VT100, and its use is recommended in almost all cases. When you create a new account, the terminal type for that account is set to vt100 by default. Just about every service computer that knows about any terminal types at all beyond the "simple" type knows about the vt100 and will be able to provide you its fairly powerful graphics capability. It is rare that you would want an account's terminal type to be anything but vt100.
The DEC VT102 is a slightly more powerful cousin of the vt100; the only diference is its escape sequences to insert and delete lines, operations which have to be simulated using an obscure feature called "scrolling regions" on the vt100. You may want to use a vt102 on an account on a service computer that understands it, such as a VMS computer - there is a small improvement in efficiency in doing so.
TOPS Terminal knows what terminal type to use for each account by looking in the relevant account description. Problems can result if the service computer hasn't been told what terminal type to expect - the service computer and TOPS Terminal have to agree on the terminal type, or else the screen can get messed up because the service computer sends escape sequences appropriate for a different terminal type.
A few service computers, like VMS, have ways to auto-magically find out whether you're imitating a DEC terminal, like the vt100, vt102, and vt52, so you don't need to do anything special on VMS and other similarly intelligent computers.
However, on UNIX, you have to explicitly tell the UNIX service computer what terminal type you're using, which ought to be vt100. To so inform it, you edit a file called ".login" or ".profile" and add a line saying respectively "setenv TERM vt100" or "set term=vt100". When your account is freshly created by the system administrator on the UNIX computer, it will have been set up with a "virgin" .login or .profile file, so just edit the one in your home directory.
It is harmless to specify a more powerful terminal type than "simple" for an account on a service computer that expects a "simple" terminal. The service computer will never send any escape sequences, so the effect will be exactly the same as if the account were set to use a "simple" terminal type.
