Meaning of "65 818 admissions were eligible for matching and 5008 were matched (1:1) in each exposure group" (medical study) From the abstract of a study that uses propensity score matching to assess the severity of adverse effects of electroconvulsive treatment (ECT):

In propensity score matched analyses, there were 10 016 psychiatric hospitalisation records (6628 women, 3388 men) with mean age 56·6 years (SD 16·3) and no ethnicity data available. 65 818 admissions were eligible for matching and 5008 were matched (1:1) in each exposure group.

I've read the Wikipedia page on propensity score matching, but I fail to understand, based on the abstract only, what this sentence means.
If the total number of hospitalizations equaled 10016, with 5008 receiving ECT and 5008 not receiving ECT, when where did the figure 65818 come from?
Or maybe I do not clearly understand the technique for propensity score matching. In my guessing, it's when a group of patients is found who did not receive the treatment despite being similar to the patients that received the treatment. Maybe I'm getting it wrong.

In propensity score matching, each exposed unit is paired with an unexposed unit, and the unpaired units are discarded. When matching with a caliper, paired units can only be within some distance of each other, and any treated units without a close enough pair are discarded. Because units are discarded when using matching, the analysis sample size after matching will be smaller than the original sample size. In this case, the hospital database or whatever data source was used contained 65818 eligible patients, but after matching, units were discarded such that only 10016 remained.
A nice, fairly accessible introduction to propensity score analysis is Austin (2011). An introduction to matching methods specifically (of which propensity score matching is one type) is Greifer and Stuart (2021).
