The Jargon File (version 4.4.7, 29 Dec 2003):

plan file
 n.

    [Unix] On systems that support finger, the .plan file in a user's home
    directory is displayed when the user is fingered. This feature was
    originally intended to be used to keep potential fingerers apprised of
    one's location and near-future plans, but has been turned almost
    universally to humorous and self-expressive purposes (like a sig block).
    See also Hacking X for Y.

    A recent innovation in plan files has been the introduction of ?scrolling
    plan files? which are one-dimensional animations made using only the
    printable ASCII character set, carriage return and line feed, avoiding
    terminal specific escape sequences, since the finger command will (for
    security reasons; see letterbomb) not pass the escape character.

    Scrolling .plan files have become art forms in miniature, and some sites
    have started competitions to find who can create the longest running,
    funniest, and most original animations. Various animation characters
    include:

    +-------------------------------+
    |Centipede:              |mmmmme|
    |------------------------+------|
    |Lorry/Truck:            |oo-oP |
    |------------------------+------|
    |Andalusian Video Snail: |_@/   |
    +-------------------------------+

    and a compiler (ASP) is available on Usenet for producing them. See also 
    twirling baton.


The Free On-line Dictionary of Computing (30 December 2018):

plan file