ISSU is supported only within and between major releases, for example, 17.3.x to 17.3.y, 17.6.x to 17.6.y (within a major release) and 17.3.x to 17.6.x, 17.3.x to 17.9.x (among major releases), that is, for two releases after the current supported release. ISSU is NOT supported within and between minor releases or between minor and major releases, for example 17.4.x to 17.4.y or 17.4.x to 17.5.x or 17.3.x to 17.4.x.
As to the original question I think it's self explanatory - which part don't you understand?
1. It is only supported in the extended support releases 17.3, 17.6, 17.9
2. You can only use it to go from within one of those releases to another dot release of that version eg. 17.3.1 to 17.3.3 or between those releases eg. 17.3.1 to 17.6.1
3. It's only supported up to 2 releases later eg 17.3.1 to 17.9.1 = supported but 17.3.1 to 17.12.1 not supported because that's 3 releases later but 17.6.1 to 17.12.1 would be supported.
But if you going to use ISSU then read the release notes very carefully and test thoroughly (to make sure it works with your network and configuration) then use at your own risk. As others have said it can often cause more problems than it solves.