This is a list of 100 10 mv stars in 0° declination taken from the UCAC5 catalog. The explanations of the UCAC5 data are given in the documentation that accompanies the catalog:
item bytes name unit description
-------------------------------------------------------------------------
1 00-07 srcid Gaia source ID
2 08-11 rag mas Gaia RA at epoch 2015.0 (from DR1, rounded to mas)
3 12-15 dcg mas Gaia Dec at epoch 2015.0
4 16-17 erg 0.1 mas Gaia DR1 position error RA at epoch 2015.0
5 18-19 edg 0.1 mas Gaia DR1 position error Dec at epoch 2015.0
6 20 flg 1 = TGAS, 2 = other UCAC-Gaia star, 3 = other NOMAD
7 21 nu number of images used for UCAC mean position
8 22-23 epi myr mean UCAC epoch (1/1000 yr after 1997.0)
9 24-27 ira mas mean UCAC RA at epi epoch on Gaia reference frame
10 28-31 idc mas mean UCAC Dec at epi epoch on Gaia reference frame
11 32-33 pmir 0.1 mas/yr proper motion RA*cosDec (UCAC-Gaia)
12 34-35 pmid 0.1 mas/yr proper motion Dec (UCAC-Gaia)
13 36-37 pmer 0.1 mas/yr formal error of UCAC-Gaia proper motion RA*cosDec
14 38-39 pmed 0.1 mas/yr formal error of UCAC-Gaia proper motion Dec
15 40-41 gmag mmag Gaia DR1 G magnitude (1/1000 mag)
16 42-43 umag mmag mean UCAC model magnitude (1/1000 mag)
17 44-45 rmag mmag NOMAD photographic R mag {1/1000 mag)
18 46-47 jmag mmag 2MASS J magnitude (1/1000 mag)
19 48-49 hmag mmag 2MASS H magnitude (1/1000 mag)
20 50-51 kmag mmag 2MASS K magnitude (1/1000 mag)
-------------------------------------------------------------------------
The columns in the Gaia vs mean UCAC coordinates table are as follows:
Gaia source ID: The Gaia identifier for the star. Item 1.
Gaia RA, dec: epoch 2015.0: Items 2 and 3.
Mean UCAC RA, dec at epoch: Items 9 and 10.
epoch: Item 8.
UCAC precessed to 2015: Items 9 and 10, precessed to 2015.0
Umag: Item 16.
Given the above documentation, one would expect that the "mean UCAC position at epoch" precessed to 2015 would be the same or very close to the Gaia position. It's not.