The mod and via controls indicate the minimum and maximum values that the modulated parameter can attain, in comparison to the default value.
In the example, the Cut(off) frequency of the filter is set to a default value of 0.50. The mod source (Env 1) drives the Cut value up from 0.50 to 0.70 during the sound attack phase and back down to 0.50 during the decay phase.
When the via source (Ctrl A) is introduced, the following occurs: when Ctrl A is at its minimum value, nothing changes; Cutoff continues to be modulated between values of 0.50 and 0.70 by the envelope (Env 1). A maximum value for Ctrl A causes the envelope generator to vary the parameter between the values of 0.50 (the default Cut value) and 0.90 (the via amount).
You can see the degree of maximum influence on basic parameters by the mod and via modulation sources—the area between the mod and via points shows the amount that the modulation depth can be further altered by the via modulation source. In the example, the Cutoff can reach values between 0.70 and 0.90, depending on the value being sent by Ctrl A.
Another example:
Logic Pro Instruments: PDF