This section defines the meanings of the terms "compliant" and "compatible" with respect to this standard, and of "partial" compliance and conformance.
An "implementation" here refers to a distribution, an installed system, a program, a package (or some similar piece of software or data), or some component thereof.
An implementation is fully compliant with this standard if every requirement in this standard is met. Every file or directory which is part of the implementation must be located as specified in this document. If the contents of a file are described here the actual contents must correspond to the description. The implementation must also attempt to find any files or directories (external to itself) primarily or exclusively in the location specified in this standard.
An implementation is fully compatible with this standard if every file or directory which it contains can be found by looking in the location specified here and will be found with the contents as specified here, even if that is not the primary or physical location of the file or directory in question. The implementation must, when it attempts to find any files or directories which are not part of it, do so in the location specified in this standard, though it may also attempt to find it in other (non-standard) locations.
An implementation is partially compliant or compatible if it complies with or is compatible with a significant subset of this document. Partial compliance or compatibility is only intended to apply to distributions and not to separate programs. The phrase "a significant subset" is admittedly subjective, and in borderline cases, the concerned party should contact the FHS editor. It is anticipated that some variation will be tolerated in borderline cases.
To qualify as partially FHS compliant or partially FHS compatible an implementation must provide a list of all places at which it and the FHS document differ in addition to a brief explanation of the reasoning for this difference. This list shall be provided with the implementation in question, and also made available to the FHS mailing list or the FHS editor.
The terms "must", "should", "contains", "is" and so forth should be read as requirements for compliance or compatibility.
Note that an implementation does not need to contain all the files and directories specified in this standard to be compliant or compatible. It is merely necessary for those files that it does contain to be located appropriately. For example, if the minix filesystem is not supported by a distribution, the minix tools need not be included, even though they are mentioned explicitly in the section on /sbin.
Furthermore, certain portions of this document are optional. In this case this will be stated explicitly, or indicated with the use of one or more of "may", "recommend", or "suggest". Items marked as optional have no bearing on the compliance or conformance of an implementation; they are suggestions meant to encourage common practice, but may be located anywhere at the implementor's choice.