This is related to the earlier one about dimensions, and I had been using SW for a few years before I found this one out! You can link dimenions so that they are always the same. right-click the dimension you want to link and select 'link dimensions' from the drop down list. Give the link some useful name and then carry on. You can cntrl-select multiple dimensions and link them all at once if you want. Interestingly the linkage isn't defined by the first dimension you pick, instead you can click any of the linked dimensions and edit it to update all of the rest. I find this useful in the following examples: - within a sketch if you are offseting multiple different sketch entities that aren't in a chain so you cant select them all at once; now you can ensure the offset dimension is the same for all of them - if you are offsetting surfaces (as a failed-shell work around for example) it can help where it wont let you offset all the surfaces in one go. This way if the offset amount changes you need only update it once. - if you want to relate a sketch dimension to a feature dimension, such as an offset line to a shell dimension There are many more instances where it's useful - these are just a couple of examples. NOTE: it doesn't work between parts in an assembly. For this you have to use the equations. You can use it for assembly features though, such as mate distances or planes etc.
