It is the only listed category defined partly by a relationship: the work must be in direct support of a professional in one of the named scientific disciplines, alongside the applicant's own theoretical knowledge and ability to solve practical problems in that discipline.
Two halves, and the second one is easy to forget
Applicants concentrate on their own qualifications and forget that the category also describes the environment the work happens in. If the American employer intends the role to run independently, with no professional in a named discipline directing or relying on the work, the fit becomes difficult regardless of how skilled the applicant is. Ask the employer plainly who the supervising professional would be and in which discipline they practise.
If the honest answer is that nobody fills that role, that is worth knowing before anything else is prepared. Where a professional is involved, get them engaged in the paperwork early, because their description of the technical relationship is the part of the file that no one else can write. Hypothetical example: Priya calibrates environmental monitoring instruments for a manufacturer of ultraviolet water-treatment systems.
A first review would map her daily calculations and fault analysis against the named scientific discipline, then identify the professional whose design or research work uses those results. The decisive record is usually that professional's technical explanation, supported by training records rather than a broad job title. Sequence the inquiry by confirming the supporting relationship before asking for letters.
A common failure is an offer that treats the applicant as an independent field contractor; that changes the analysis because this classification cannot be used for substantive self-directed business activity.