OPENTBS_SELECT_FOOTER

$TBS->PlugIn(OPENTBS_SELECT_FOOTER, [$Type[, $Offset])

in what way I can edit the footer in the documents? to say I found this function but do not know how I use it, you can some example?

If you are using an LibreOffice template then header and footers are part of the main contents. So there is no particular operation and command OPENTBS_SELECT_FOOTER will do nothing and will not produce any error.

If you are using an MsOffice template then footer are save in a separated sub-file. So the command OPENTBS_SELECT_FOOTER helps to select the correct sub-file and then you can continue the merging as if is was a normal contents.
Parameters $Type and $Offset can be used for loading advanced footers. This is because Ms Word can manage footers for first page, add pages, and can also manage different footers for some pages.

And in what way can I zmienc footer? I have a different name in MYSQL and choose which generates a file and footer should be a change in something like:

$ Footer = text

opentbs plugin footer = $ footer

I used Libreoffice only

> I used Libreoffice only

With LibreOffice documents, simply manage your TBS fields inf the Footer just like the main contents.
Command OPENTBS_SELECT_FOOTER has no effect with LibreOffice.

Command OPENTBS_SELECT_FOOTER does not merge any content. It simple select the Footer content to prepare a new merging if the Footer contents is in a separated sub-file.