The Principal Prohibition that was in the Mishkan: Tying and untying are amongst the principal [forbidden] actions, being that in the [process of building the] Tabernacle, the fishermen for the chilazon would tie and untie their nets. As all nets are made of many knots and these knots are permanent, and at times [the fishermen] would need to remove a rope from one net and add it to another net and [he would thus] untie it from [the first net] and [then] tie it [on the second net], [and thus we see that there existed both tying and untying in the works done for the Tabernacle].
One is only liable on a permanent knot? Biblically one is only liable [on making] a permanent knot which is made to last forever, which means that he ties it with intention that it remain [tied] as long as it is able to exist and as long that he does not need to untie it.
The reason behind the above definition of “permanent”: Now, although it is possible that he [may] need to untie it in a [very] short time and will thus untie it, nevertheless since when he ties it he does not set a date in his mind for when he will un-tie it, and it is [thus] possible that it will remain [tied] forever, [therefore] it is considered a permanent knot, and one is liable on tying it and on untying it.
First Opinion-No difference between a professional and amateur knot:  Some opinions rule that there is no difference at all between a professionally made and amateur made [knot], as even on an amateur knot one is Biblically liable if made to permanently last, and if [a knot] is designated to not last at all [then] in a situation that it is [thus] allowed to make an amateur knot then it is also allowed to make a professional knot.
Second Opinion-Biblical liability is only applicable by professional knots:  There are opinions which say that one is [never] Biblically liable even on a knot that is meant to last forever unless it has been professionally done, which means that it is a strong and tough knot similar to the knots that the professionals make during their work, such as the knots of the straps for shoes and sandals which the strap makers tie at the time of making them, and the same applies for all similar cases. However one who ties a permanent knot which is not professionally done is exempt [from liability].
For example, a strap broke and one tied it, [or a] rope tore and one tied it, or one tied a rope to a bucket or one tied an animal leash [to an animal] and so too all cases [of knots] similar to these knots which are made by amateurs [then] any person who ties them permanently is exempt [from liability].
Is it Biblically forbidden to untie a knot when done without intent to re-tie?
Yes. Thus a permanent knot may never be untied on Shabbos even through a gentile, even in a case of need.
May an amateur permanent knot be untied by a gentile in times of need?
 317/2; So rules Rosh, Rashi, Tur and Rama
 317/2. So rules Rif, Rambam [Shabbos 10/1] and Michaber
 So rules Taz. However see Biur Halacha which learns that a professional knot is defined as a knot which will not be undone on its own, even if it can be undone with a single hand. The Nafka Mina between these two opinions is regarding making a loose double knot to untie on the same day, to which according to Admur would be permitted as opposed to in accordance to the Mishneh Berurah.
 Vetzaruch Iyun as in the Mishkan such knots were made with intent to re-tie?
 Peri Megadim M”Z 317/5
 As this matter is disputed whether it contains a Biblical prohibition. This applies even if one were to hold that a Biblical doubt is only Rabbinical, as it nevertheless has a Torah root. [ibid]