The phrase "only part" means that something is only a portion or segment of a larger whole. It suggests that what you are referring to is not the complete thing, but rather just one aspect or element within it.
The second step is to match these lots with sales, which may divide them into smaller increments, because a particular sale transaction may dispose of only part of a lot. (fairmark.com)
But I accept this as not only part of my journey, but as part of my mission. (health.com)
If you're paying for only part of your purchase with the card, only the portion charged will be eligible for the discount. (cardrates.com)