From the title, you seem to be asking from a music theory / composition point of view, but looking closer, you're really asking a programming question? If you are just trying to parse the data (which really should be a StackOverflow question), you should just look into how to parse MIDI data. Looking at steps and beats will not help you much as that is just an abstraction meant for composers. But for those coming for the title, here is a full explaination:
A 'step' is basically just a position in a sequence. If you look at a classic TR-808 drum machine, it has a 16-step sequencer. That's 4 steps per beat, 16 steps per bar (or measure), and that's the standard way to divide steps into a measure with a 4/4 time signature. Of course, modern sequencers let you change this granularity. You can go all the way to 16 or 24 steps per beat, at extreme granularity.
FL Studio's granularity is controlled by PPQ - Pulses per quarter note, which is set to 96 by default, but can go up to 960. That means 960 'pulses' (or ticks) per beat. So that's the connection.
One beat is one quarter note. One bar is a whole note. However, a quarter note is not always a beat, which is depending on the time signature. So essentially step-sequencers are built around the premise of 4/4.
This PPQ system was introduced by the MIDI standard in the mid 80s because they needed enough granularity to capture human performances in all their errors. As an interesting side note, Quantization is the act of aligning notes to the nearest step in a given granularity, eliminating human error, but this ruins the human element of a skilled performer. Only robots have perfect accuracy to the millisecond.
So to answer your question, the length of a step is equivalent to these note values (but not always), where a measure must add up to a whole note. Also remember this is also dependent on BPM (Beats-per-minute) of course.
I'm no expert in music theory (and this is not the music theory SE) but sheet music is not a science; it's meant for humans (musicians) to read and perform the music to a certain degree of confidence, not perfectly recreate a piece exactly how it was intended. It's what allows for improvisation. When performing or transcribing sheet music, the main idea is there, but it can be interpreted in different ways.
DAWs and MIDI sequencers are precise, almost robotic. With enough granularity, you can capture any performance digitally and recreate it exactly. Or of course, sequence it manually.
Something like the Roli Seaboard probably has so much expressive capacity, that some things just can't be transcribed unto sheet music. It's all in the improv and performance detail.
I can only suggest diving deep into music theory / music reading / classical composing to truly combine the disciplines and understand how they complement each other. I'm afraid there is only so far you can go without understanding and being able to perform sheet music yourself.