ASTM E18-19 1

As hardness increases, the size of the indent gets smaller and you need less material to make sure you're not poking through the back into your anvil. So a soft material needs more thickness to minimize any errors caused by going through the test material. A harder material is by definition more resistant to the penetration and needs less material to avoid getting errors from the anvil.

OP: your knowledge is correct, but your understanding of table A5.1 is wrong. table A5.1 specifies the scale (A or C) to choose when using DIAMOND indenter.

For example, when thickness is .030, your material needs to be 57RC or above to use diamond indenter and scale C. If hardness is less than 57RC, you should use a different scale and/or a different indenter, WC ball, say scale A, diamond. It does not say, if hardness is higher than 57RC, the thickness must be less than .030''.

The same is true: AidanMC's explanation makes sense, but seems irrelevant.

AidenMc's explanation is highly relevant. Very often samples received from customers are too thin to determine conformance in the specified scale, in which case we will have to change scale (for example, from Rockwell B to superficial hardness) as thickness by table A5.1 is below minimum required to prevent full penetration of the indenter. Sometimes, this will mean using microhardness per ASTM A384 instead and making a mount.

You are basically repeating what OP and AidenMc have said, but did NOT answer the question. I believe OP got your point, and that was exactly why he/she raised the question.

Table A5.1 is only part of the equation. It covers direct Rockwell A and C testing. Table A5.2 applies to Rockwell F and B indenters, while Table A5.3 covers superficial HR15N, HR30N, and HR45N hardness. Table A5.4 similarly covers minimum thickness requirements for HR15T, HR30T, and HR45T scales. Consult the appropriate table to determine if you need to change the applied hardness test.