Yes, it implies that part of the string or the whole string has no mix-ups aside from free cancelling (i.e. a static string).
For example, Rachel's 3PPP is a fully static string since all of them are mid punches and she has no strike mix-ups. Rachel 4PP is static because the second one is always a mid punch, but after that the string becomes dynamic because she has two different mix-ups: 4PPP and 4PP2P. So Rachel's 4P string is partially static (and partially dynamic).
As another example, Kasumi's 1PP is fully static because she has fixed strikes and can't mix up with anything. It's always a low to high, no matter what. But Kasumi's 6P is very dynamic since she has many different follow-up on different hit levels: 6PP (mid punch - high punch), 6PK (mid punch - mid kick), 6P6K (mid punch - mid kick) and 6P2K (mid punch - low kick). Even 6PK is dynamic, because she has 6PKK (mid punch - mid kick - mid kick) and 6PK2K (mid punch - mid kick - low kick).
Momiji's 3K string is another example of a partially static string. After 3K, there is always a mid punch if the Momiji player decides to continue the string so the opponent can anticipate the mid punch and hold it. But if they don't hold that punch, she has access to different mixups from her Uzukaze stance and the string becomes dynamic. So 3KP is static, but after that the string becomes dynamic because then the mix-ups come into play.
We as players search for static strings so we can safely say what follow-up a certain strings leads to. The defending player only has to worry about a free cancel when it comes to a static string, which can be reacted to with a punish or counter hit, depending on the situation. If the offending player decides to continue the static string, the defending can be 100% sure of the hit level of the strike and hold it. It's a good way to exploit character weaknesses.
TL;DR
Fully static string = The whole string is composed of fixed strikes. There is no room for mix-ups except for free cancelling the string. (e.g. Rachel's 3PPP, Kasumi's 1PP)
Partially static string = A part of the string is static (i.e. no mix-up potential in terms of strikes) but another part - usually the later part of the string - is dynamic (i.e. there is room for mix-ups) (e.g. Rachel's 4P string, Momiji's 3K string)
Dynamic string = The whole string has mix-up potential (e.g. Kasumi's 6P string).
I hope this clears up what I said before.