Here is a comparison of the length of IVSC to USPAP in Pages, Words, Characters, Paragraphs, Lines
That which is not prohibited is generally allowed.
No matter which appraisal code of conduct we're talking certain elements will always be prohibited when someone is engaged in appraisal practice
client advocacy
biased results
lying cheating or stealing in an appraisal
criminal conduct
gross negligence
misleading reports
incompetent performance
So any client request that includes any of the above will be prohibited regardless of which code or standard we're talking about. Some of the housekeeping items will vary like (in USPAP) the additional requirements for records retention or prior services disclosures or whatnot, but in terms of the basics of development and reporting there will be a lot of overlap.
As for either prohibiting or requiring "a grey area THAT IS AT THE DISCRETION OF THE APPRAISER" to which you refer, perhaps you can provide an example of what you're talking about that isn't already addressed in USPAP as it stands.
To repeat again, this goes back to what I said about being able to extract a part of the standard and use it in isolation to support unacceptable behavior. I don't see this (so far) in IVSC. IVSC doesn't make many of the mistakes that USPAP makes in technical issues related to using the standard.
[Note: A detailed comparison of IVS to USPAP shows that USPAP is 2.97 times longer (189,830 words vs 63,860 words for IVS), while IVS nonetheless deals with many things in more detail, such as defining 6 bases of value. The people that wrote it have a good technical understanding of appraisal and finance and are schooled in logic (in particular I see the influence of Material Logic (aka Aristotle's "Posterior Analytics "). The only good thing about USPAP is that, in order, (1) The punctuation is superior and (2) the spelling somewhat better, according to MS Word; but that is likely due to the influence of English spelling and contributions by foreigners for whom English is only a second or third language. The bad punctuation you also see in a lot of technical writing here in the US, especially in Biotech.]
A previous example I have used on this forum in discussions was the fact that the Acceptability Comment in the SOW Rule of USPAP states, as an entire statement that can be extracted and used in isolation to prove whether something conforms to SOW, that conformance to USPAP is NOT required for acceptability:
This creates a contradiction in statements. IVS has (a)-

parts to the SOW requirements and that last one

states that the SOW must comply with IVS. And, please don't repeat the nonsense about peers, - at term which doesn't exist in IVS.
IF your first statement above was correct, then we would only need statements of prohibition in a standard. That would be a pretty contorted standard that doesn't make any sense. A standard is a Social Contract, a formal agreement to do things a certain way, -- WITH many conditions.
Microsoft Word Analytical Comparison of IVS and USPAP, Metrics and Grammar
| Metric | IVSC (2020) | IVSC Ethics | IVSC Review | IVSC Total | USPAP | RATIO USPAP/IVSC |
| Pages | 141 | 15 | 33 | 189 | 402 | 2.13 |
| Words | 50,409 | 4,370 | 9,081 | 63,860 | 189,830 | 2.97 |
| Characters | 273,756 | 25,058 | 49,437 | 348,251 | 942,777 | 2.71 |
| Paragraphs | 2,731 | 165 | 334 | 3,230 | 9,579 | 2.97 |
| Lines | 8,863 | 520 | 1,273 | 10,656 | 16,453 | 1.54 |
| Average Word Length | 5.4306969 | 5.73409611 | 5.44400396 | 5.45335108 | 4.96642786 | |
| Average Paragraph Length | 18.458074 | 26.4848485 | 27.1886228 | 19.7708978 | 19.8173087 | |
| Tot Errors/Suggestions | 1,127 | 111 | 263 | 1,501 | 3,817 | 2.54 |
| Spelling | 242 | 32 | 14 | 288 | 425 | 1.48 |
| Grammar | 227 | 26 | 218 | 471 | 1,211 | 2.57 |
| Clarity | 381 | 34 | 16 | 431 | 1,213 | 2.81 |
| Conciseness | 135 | 10 | 4 | 149 | 371 | 2.49 |
| Formality | 2 | ok | ok | 2 | 24 | 12.00 |
| Punctuation | 50 | 1 | 6 | 57 | 38 | 0.67 |
| Resume | 67 | 7 | 2 | 76 | 387 | 5.09 |
| Vocabulary | 23 | 1 | 3 | 27 | 148 | 5.48 |
By the way, the TAF and IVSC have a document on writing appraisal reports to comply with both the IVS and USPAP. IVS preempts USPAP in most cases, as far as I see, because it is more strict. For example, although I put my Limiting Conditions in the Engagement Agreement, most US appraisers don't do that. That would be required by IVS.
Also, as you might expect, local laws and regulations preempt anything else in IVS as "departures", but apparently we don't have to worry about a conflict between USPAP and IVS - as long as we use the IVS terms instead of the USPAP terms (IVS requires the use of their terms, whereas USPAP does not. .... Thus "Special Assumption" rather than "Hypothetical Condition", although why not use both?)