A lockout is generally used to enforce terms of employment upon a group of employees during a dispute. A lockout can act to force unionized workers to accept changed conditions such as lower wages. If the union is asking for higher wages, or better benefits, an employer may use the threat of a lockout or an actual lockout to convince the union to back down.
“In the United States , under federal labor law , an employer may hire only temporary replacements during a lockout. In a strike, unless it is an unfair labor practice (ULP) strike, an employer may legally hire permanent replacements. Also, in many U.S. states, employees who are locked out are eligible to receive unemployment benefits , but are not eligible for such benefits during a strike.
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Most strikes are undertaken by labor unions during collective bargaining . The object of collective bargaining is to obtain a contract (an agreement between the union and the company) which may include a no-strike clause which prevents strikes, or penalizes the union and/or the workers if they walk out while the contract is in force. The strike is typically reserved as a threat of last resort during negotiations between the company and the union, which may occur just before, or immediately after, the contract expires.
“United States labor law also draws a distinction, in the case of private sector employers covered by the National Labor Relations Act , between "economic" and "unfair labor practice" strikes. An employer may not fire, but may permanently replace, workers who engage in a strike over economic issues. On the other hand, employers who commit unfair labor practices (ULPs) may not replace employees who strike over ULPs, and must fire any strikebreakers they have hired as replacements in order to reinstate the striking workers.”

• Economic strikers defined. If the object of a strike is to obtain from the employer some economic concession such as higher wages, shorter hours, or better working conditions, the striking employees are called economic strikers. They retain their status as employees and cannot be discharged, but they can be replaced by their employer. If the employer has hired bona fide permanent replacements who are filling the jobs of the economic strikers when the strikers apply unconditionally to go back to work, the strikers are not entitled to reinstatement at that time. However, if the strikers do not obtain regular and substantially equivalent employment, they are entitled to be recalled to jobs for which they are qualified when openings in such jobs occur if they, or their bargaining representative, have made an unconditional request for their reinstatement.

Example:
If the strike is deemed an economic strike, if the
Company decides to hire 100 "permanent replacements" and after the strike, they said, we need 715 employees back out of the 750, than the
35 least senior people would be given their jobs back as openings/retirements occur.

• Unfair labor practice strikers defined. Employees who strike to protest an unfair labor practice committed by their employer are called unfair labor practice strikers. Such strikers can be neither discharged nor permanently replaced. When the strike ends, unfair labor practice strikers, absent serious misconduct on their part, are entitled to have their jobs back even if employees hired to do their work have to be discharged.
• If the Board finds that economic strikers or unfair labor practice strikers who have made an unconditional request for reinstatement have been unlawfully denied reinstatement by their employer, the Board may award such strikers back-pay starting at the time they should have been reinstated.

The Right to Strike
• Section 7 of the National Labor Relations Act states in part,
“Employees shall have the right. . . to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection.” Strikes are included among the concerted activities protected for employees by this section. Section 13 also concerns the right to strike. It reads as follows:
• Nothing in this Act, except as specifically provided for herein, shall be construed so as either to interfere with or impede or diminish in any way the right to strike, or to affect the limitations or qualifications on that right.
• It is clear from a reading of these two provisions that: the law not only guarantees the right of employees to strike, but also places limitations and qualifications on the exercise of that right.