http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/seek.htm

Example (no error handling done here - be sure to do some in real life!) :

 % cat /tmp/mydata
 1234567890abcdefghijklmnopqrstuvwxyz
 % set f [open "/tmp/mydata" "r+"]
 file3
 % seek $f 20
 % puts -nonewline $f "KLM"
 % close $f
 % cat /tmp/mydata
 1234567890abcdefghijKLMnopqrstuvwxyz

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But what if the file is not a plain [ASCII] file, but instead a binary file?
Well, there are two trains of thought here.  One is that, rather than
inserting/replacing at various seek spots within the file the new data,
one should be working in terms of the data structures within the file, to
ensure that the changes do not result in an invalid file.

The other approach is that ''if you know what you are doing, then do it''.
There is one difference however.  Before the invocation of whatever output
file is to be done, one needs to make certain that the [file handle] 
(the return value from [open], etc.) is configured to work with binary data -
otherwise, [Tcl] may attempt to apply character [encoding] changes to the outgoing data.

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Is there supposed to be a difference in the seek/[puts] behavior based on varying the open access value between "r+, "w+" and "a+"?

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[A little fortune teller] has another usage example with random seek.

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[[seek]]ing past the end of a file has operating-system-specific consequences,
including, in some cases, "gap-filling with nulls".
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[Tcl syntax help] - [Arts and crafts of Tcl-Tk programming] -
[Category Command] - [Category File]