I need help to understand the legal value of this sentence from a contract: " Contractor will invoice CRO monthly" and "CRO shall send payment for services in X days". "Will" and "shall" have the same legal value of "must" or using "will" doesn't create e legal obligation? Does "shall" create a legal obligation? And the second one I'm in doubt: "During months where no time on site is required, time spent on this project will be capped at 20 hours". According to you, the meaning is that " during months with no site activity, one can bill a maximum of 20 hours" or "The word “capped” does not mean that you have the right to invoice for 20 hours"?
Does "shall" create a legal obligation?
Typically, yes. This query retrieves case law in the sense that "shall" is mandatory. In the clause you mention, "shall" precludes the client's possible choice of delaying payment for more than X days.
There is some case law making "will" synonymous to "shall". For instance, see Briggs v. State, 226 So.3d 59, 66 (2017) ("Like shall, will is mandatory*", italics in original). But it is important to be mindful that "will" may reasonably have an informative purpose rather than unequivocally creating a duty, especially when an interpretation of duty leads to an absurd or very questionable outcome.
In your clause, it would be quite a stretch to interpret "will" in the sense of "contractor forfeits his entitlement to payment if he does not bill on time". Instead, the language "Contractor will invoice CRO monthly" informs CRO the frequency with which CRO is to be billed, meaning only that the contractor is not supposed to bill CRO --say-- weekly.
Answering the issue of time being capped at 20 hours requires more information about the contract and/or the context. At first glance, it seemingly suggests that the contractor is not allowed to bill more than 20 hours if during a billing period no work took place on site. On the other hand, it could mean that the client should not expect the contractor to make great progress (that is, more than the equivalent of 20 hours of effort) if no work took place on site. Thus, discerning the restrictive sense of "capped" depends on more than the excerpt you reproduced.