Are There Exceptions to Who Needs to Submit Form I-864?

According to Instructions for Affidavit of Support Under Section 213A of the INA, The following types of intending immigrants do not need to file Form I-864: "Any intending immigrant who has earned or can receive credit for 40 qualifying quarters (credits) of work in the United States. In addition to their own work, intending immigrants may be able to secure credit for work performed by a spouse during marriage and by their parents while the immigrants were under 18 years of age. The Social Security Administration (SSA) can provide information on how to count quarters of work earned or credited and how to provide evidence of such. See the SSA website at https://www.ssa.gov/myaccount/ for more information;"

2 questions here:

1) If the parents worked in the US for more than 40 quarters (combined, under H1B visa) and then left the US, these 40+ quraters (earned in the past) can be used to examp the affidavit requiremetns when they want to be sponsorded now by their 21 years old child, a US citizen? 

2) How to understand "can receive credit for 40 qualifying quarters (credits) of work in the United States." What "can" (not "has earned") mean? If the parents both are TN visa holders and both get full time permanent job with dencet pays (say combined more than $120,000 per year in the middlewest area)under TN status/visa, does that meet "can" requirements? 

Thanks always.