Scaled

Scaled[{x,y,…}] gives the position of a graphical object in terms of coordinates scaled to run from 0 to 1 across the whole plot range in each direction. 

Scaled[{dx,dy,…},{x0,y0,…}] gives a position obtained by starting at ordinary coordinates {x0,y0,…}, then moving by a scaled offset {dx,dy,…}. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.