Skill in welding, even at an advanced level, is general expertise. It may qualify as specialized knowledge only where the file shows special or advanced knowledge of the group's own procedures, qualifications or products; L-1B does not require proof that the knowledge is unavailable in the U.S. labour market.
Which document decides it
The deciding document is the company's own procedure library. Hypothetical example: a welding engineer at a Lévis shipyard is asked to transfer to the group's U.S. yard to qualify its welders on the group's proprietary procedure specifications for a particular steel and joint design.
If those procedure specifications exist as controlled internal documents, if the engineer wrote or revised them, and if the U.S. yard cannot qualify welders on them without her, the knowledge is the company's and hers. If, instead, she would be applying industry-standard codes that any qualified welding engineer knows, the same résumé describes ordinary expertise.
Ask the employer to produce the procedure documents, the revision history showing her involvement, and a statement of why the U.S. yard needs her rather than a local hire. The petition is built from those three items, and if the second cannot be produced the claim is weaker than the company believes.