electrical length degrees
In an simulation program, an ideal electrical length is given as a number of degrees of phase and the frequency that the phase is measured at. Such as "45 degrees at 2 GHz". That assumes that the dielectric constant is 1.0 and that the electrical length varies linearly with frequency.
However, since air dielectrics are seldom encountered in a circuit design, a lot of programs allow you to define electrical length as a physical length and a dielectric constant. Such as "0.1 meter and an effective dielectric constant of Er=4.5". That, once again, assumes that the electrical length varies linearly with frequency.
That is about it. In one of the most common media, microstrip, the electrical length does NOT vary lineary with frequency (called dispersion). So when you enter a length of a microstrip line, and the substrate dielectric constant, the program computes an effective dielectric constant, and thus an electrical length, that is a function of Physical length, substrate dielectric constant, and frequency.