The first is the request to do the job and the second the authority once the team has satisfied the safety regulations.

In the situation you describe, a work order normally fulfils both functions: assigning the work to be done and authorising the persons to whom the order is issued to do the work (as a matter of principle, work orders are not issued to persons or organisations that are not qualified to do the work).

sample refs:

Work order (WO) - WO is a device in the work cycle that authorizes, assigns, and tracks the completion of work within an organization.
http://www.polar.bz/polarportal4p/default.aspx/MyWiki/Projec...

37-4-501. Work order for construction or repair of appliances. (1) A licensed dentist who employs or engages the services of a person, firm, or corporation to construct, reproduce, make, alter, or repair bridges, crowns, dentures, other prosthetic appliances, surgical appliances, or orthodontic appliances shall furnish the person, firm, or corporation with a written work authorization on forms prescribed by the board which shall contain:
(a) the name and address of the person, firm, or corporation to which the work authorization is directed;
(b) the patient's name or identification number, but if only a number is used the patient's name shall be written on the duplicate copy of the work authorization retained by the dentist;
(c) the date on which the work authorization was written;
(d) a description of the work to be done, including diagrams if necessary;
(e) a specification of the type and quality of the materials to be used; and
(f) the signature of the dentist and the number of his license to practice dentistry.
(2) The person, firm, or corporation receiving a work authorization from a licensed dentist shall retain the original work authorization and the dentist shall retain the duplicate copy for inspection at a reasonable time by the board for a period of 2 years from date of issuance.
http://data.opi.state.mt.us/bills/mca/37/4/37-4-501.htm

I translated documents relating to this issue, and picked some to illustrate it.

International Association of Oil and Gas Producers Publication
"A P.T.W. is not simply a permission to carry out a dangerous job. It is an essential part of a system which determines how that job can be carried out safely.
The terms "P.T.W.", "permit" or "work permit" refer to the certificate or form which is used as part of an overall system of work..." http://www.ogp.org.uk/pubs/189.pdf

PERMIT-TO-WORK FOR UNIVERSITY MAINTENANCE STAFF ORCONTRACTORS REQUIRING ENTRY INTO LABORATORIES AND/ORASSOCIATED AREAS
www.safety.ed.ac.uk/resources/General/Lab_permit_to_work.pd...
WORKING DOCUMENT
PERMIT TO WORK
A Permit to Work system is a formal safety control system designed to prevent accidents including injury to employees, contractors and third parties as well as to property, particularly when work with a foreseeable high hazard content is undertaken. The Permit sets out the work to be done and the precautions to be taken. Documentation: Permit to Work Notice to Contractors Control Record Procedure: 1. The Site Services Manager controls all Permits to Work on the sites under his / her control. 2. Persons selected to authorise Permits to Work must possess sufficient expertise to fully assess the nature of the likely hazard and to stipulate the precautions needed to effectively reduce the hazard to its lowest possible level. 3. Whilst it is responsibility of the contractor to advise his own personnel, the authorising person must ensure: a)That all other persons who need to know that the work is being undertaken are informed including staff/union consultations have been carried out where appropriate. b) That each such person is aware of what he must do and what action is to be taken in the event of an emergency. 4. The Permit shall include: a) Concise and accurate information about who is to do the work, the period the work is to be undertaken and the precautions required when carrying out Hot Work, Electrical Work, Working at Height, Work in Confined Spaces and High Risk Activities.
http://www.candi.ac.uk/downloads/about/procurement/permit_to...

PERMIT TO WORK form
www.qub.ac.uk/.../HumanResources/OccupationalHealthandSafet...