The Supreme Court's decision upholding seniority systems that perpetuate the effects of past job discrimination will have only limited impacton cases before the Equal Employment Opportunity Commission, its chairman said yesterday.

Attempting to clarify what she called "confusion" over the May 31 decision, Chairman Eleanor Holmes Norton issued a statement spelling out how the commission will interpret the ruling in processing job bias cases, including the 123,000 complaints now pending before it.

Although some court critics have said the ruling will cripple efforts to remedy the effects of past job discrimination, Norton said the commission will act on the assumption that it "applies to seniority only . . . and then only narrowly."

Abner Sibal, EEOC general counsel, added that "probably" most cases now in the commission's pipeline will be unaffected by the ruling. Sibal and the commission also took the position that all existing agreements reached through conciliation are valid.

In its May decision, the Supreme Court held that seniority systems adopted without intent to discriminate before passage of the 1964 Civil Rights Act are valid, even though they are based on previous employment systems that discriminated against minorities or women.

Norton said the EECC interprets this to affect only seniority systems adopted before the law took effect July 2, 1965, and for which "the evidence shows that there was no discriminatory intent in the genesis or maintenance of the system."

Both "intent" and "maintenance" appear to be key words.

In determing intent, Norton said, the commission will consider "all available evidence," including collective bargaining history and employment practices. When seniority systems were established by segregated local unions, a discriminatory intent will be "inferred," she said.

Moreover, when employers and unions know that a seniority system is discriminatory and retain or renegotiate it without change, they will have to prove to the commission's satisfaction that they had no alternative, Norton said.

"When a unit [or department-by-department] senority system is in effect and the employer or union is made aware that it is locking in minorities or females, discriminatory intent will be inferred if the system is maintained or renegotiated when an alternative system is available," the EEOC statement said.

Employers or unions are presumed to be aware of discriminatory effect if grievances or other charges have been filed, according to the statement.

In determining whether a person has been discriminated against in job transfers, the commission said a formal application for transfer need not have made. The commission said it will also consider informal inquiries, a worker's efforts to develop new job skills and previous failures on the part of employers or unions to end discriminatory employment patterns.