XS_CONNECTING_SIDE_MARK_SYMBOL
Category in Advanced options dialog: Marking - parts
Use this advanced option to change the symbol for a connecting side mark in single-part, cast unit, and assembly drawings. By default, the side mark symbol is the symbol number 34 in the xsteel.sym symbol file. To change the symbol, set this advanced option to a different symbol number.
Symbol 34 in xsteel.sym:

To define a connecting side mark symbol for general arrangement drawings, set the advanced option XS_GA_CONNECTING_SIDE_MARK_SYMBOL to the desired value in an initialization (.ini) file.
To define the minimum distance of the connecting part from the main part so that when the distance is larger than the value you enter, Tekla Structures draws a connecting side mark, set a value for the advanced option XS_MIN_DISTANCE_FOR_CONNECTING_SIDE_MARK.
For more information about connecting side marks, see Indicate part orientation.
This advanced option is model specific and the setting is saved in the options database.