This is very important, because it can allow/prevent players from "saving" XP to spend it to buy skills from higher ranks.
From Ranks (Rogue Trader Core Rulebook, p38):
Your Rank is a general measure of your experience and capabilities. It represents the progression of your character’s abilities as he grows in wealth, power, and status. Your Rank is determined by the total amount of Experience Points your character has spent.
From this, your PCs need to spend the XP in order to go up in Rank, and saving it up does not in itself allow for increases in Rank; they can, however, save up the XP, then plan exactly how they're going to go through the Ranks, buying advances per Rank until they've spent sufficient XP to reach the next Rank.
I have a starting character (5,000 XP), and have somehow survived long enough to accrue an additional, unspent, 6,000 XP, for a total of 11,000. This would put me at Rank 3 if I were determining Rank by total XP rather than total spent XP.
I first have to spend 2,000 XP at Rank 1 (this can include statline advances, in addition to everything in Rank 1) before I can purchase any advances in Rank 2.
I then have to spend 3,000 XP at Rank 2 (this can include statline advances, in addition to everything in Ranks 1 and 2) before I can purchase any advances in Rank 3.
I then have the remaining 1,000 XP to spend at Rank 3, or to save up until I have more XP to spend. This can include statline advances, in addition to everything in Ranks 1, 2 and 3.
Extend the logic however many Ranks you need and however much XP the character has, but that is a basic explanation of the steps involved.
This approach does, however, lead to a potentially vastly underpowered character (or, if they're all doing it, party) which either forces a change in the progression of the campaign (difficulty wise) or a quick death for the character(s) in question. As a GM I'd advise against saving it up in order to keep the campaign progressing, and having the characters actually develop, not just as statlines, but as actual characters.
Ranks on Warhammer 40k series of RPG are earned by spending experience.
The character progression is molded on top of acquiring a few things of a lower rank (showing training, study, or time spent amongst your peers) to unlock things from a newer rank.
When you go up a rank you start to have access to the new skills of the rank and the skills from the prior ranks, so they don't need to save up exp, just plan to spend in ranks that don't add much to the concept of the character.