The qualifying employment abroad and the knowledge the employee holds still have to be established, along with the fit between that knowledge and the proposed work. The blanket settles the corporate relationships among the listed entities; it says nothing about whether a particular person qualifies for the classification.
The convenient route is not a lower standard
Employers sometimes read a blanket approval as pre-clearance for staff and prepare thinner evidence as a result. The individual elements are unchanged, and the blanket category's professional requirement can make the employee's own qualification harder to establish rather than easier. Prepare the employment history and the knowledge description to the same standard either way, then decide which route the evidence actually supports.
Two substantive points survive whichever route is used. The knowledge showing rests on either special knowledge of the employer's products, services or their application, or an advanced level of knowledge of the employer's processes and procedures, and neither limb is satisfied by long service or general competence in a widely available technology. And L-1B stay is capped at five years in total, with time already held in that classification counting toward the limit rather than resetting.
A blanket arrangement changes where and how a case is presented; it changes neither of those. Hypothetical example: an employer holding a blanket approval prepares a two-page description for a chromatography method transfer specialist on the basis that the heavy lifting has already been done, and the description that would actually be needed — naming the employer's own method transfer protocols and the studies that shaped them — has to be written anyway.