Unlike labels, relative jumps are, as the name suggests, relative to the place they are called from. You can use relative jumps wherever you can use labels. Relative jumps are marked by numbers. +1 jumps to the next instruction (the default advancement), +2 will skip one instruction and go to the second instruction from the current instruction, -2 will jump two instructions backward, and +10 will skip 9 instructions, jumping to the tenth instruction from the current instruction.
An instruction is every command that is executed at run-time, when the installer is running. \R{messagebox}{MessageBox}, \R{goto}{Goto}, \R{getdllversion}{GetDLLVersion}, \R{FileRead}{FileRead}, \R{setshellvarcontext}{SetShellVarContext} are all instructions. \R{saddsize}{AddSize}, \R{ssection}{Section}, \R{ssectiongroup}{SectionGroup}, \R{ssectionend}{SectionEnd}, \R{asetoverwrite}{SetOverwrite} (and everything under \R{flags}{Compiler Flags}), \R{aname}{Name}, \R{asetfont}{SetFont}, \R{langstring}{LangString}, are not instructions because they are executed at compile time.
Note that \R{insertmacro}{macro insertion} is not considered as one instruction when it comes to relative jumps. The macro is expanded before relative jumps are applied, and so relative jumps can jump into code inside an inserted macro. The following code, for example, will not skip the macro. It will show a message box.
