Hi Xavi García,
Jon wrote the most important facts I think, so just a few more comments:
To understand what floating point number are, how they are implemented and what risks and threats exist there is mandatory knowledge for any CAD computation in my opinion. No deep detail experience is rewquired, just to understand basics and accept the most imporant things (like "test numbers equality is nonse ;-) and to know best practices. Wiki page mentioned by Jon is good start, but you can find plenty of articles and blogs on Internet (e.g. this one).
[quote user="Xavi García Maranges"]When I have two elements connected between them vertex to vertex or vertex to any point of the another line , the distance must be = 0 [/quote]
No, no and once more no ;-) ... well, in theory on paper the distance is zero, but the core question is how it's implemented in a particular software plus how it's maintained (or crippled) by API. And because no floating point number is exact, is e.g. 1E-10 still valid value or it's zero? If your master units are meters, if working in GIS and cadastre, anything under 1 mm is "zero" because it's below precision how data are captured. So 1E-4 (0.1 mm) can be always evaluated as 0 ... but it's not how simple API works.
[quote user="Xavi García Maranges"]like configure the work units or similar [/quote]
From general perspective, if any code depends on units configuration, it's bad and fragile code.
If you work with coordinates comming from differenct source (e.g. two elements), you should not use "simple math methods" like Point2dEqual, but Point2dEqualTolerance method, because it allows to define what is "below reasonable precision". Or you can implement own methods.
BTW It can be valuable to check some API or solution where precision and difference between math and implementation is important. I am talking about topology and GIS libraries, because any topology is always calculated with "what is enough precision" in mind and it's reflected in API also.
[quote user="Xavi García Maranges"] ComputeMinimumDistance of the BsplineCurve[/quote]
As Jon wrote, BSpline is calculated element and it cannot be treated in the same way as e.g. line or line string.
With regards,
Jan