What does this symbol mean

Record keepers used various symbols in the margins in registros and padrones and their usage was not uniform or consistent. In some instances the record creator shared with the reader what those symbols represented but often they did not, and so we may never know for sure their intended meaning.

In the second example you shared in this thread, the baptisms on image 480, the cross might represent that the individual later received another sacrament in the parish, such as the sacrament of matrimony. Or, perhaps it means that the baptism was performed as an act of charity (de limosna). I say this because a few individuals in the registro have sums above their names, like the 100 for María Franca. de la Luz española de la Barranca de las Viudas on image 485.

In your other example, from the padrón, we know the meaning behind the cross in the margin because the writer stated on the first page of the padrón the following:

“. . . el Padrón de esta Feligresía de Ayo el Chico: Los que van con Cruces han cumplido con los preceptos anuales (confesión y comunión), y los q[u]e no son por ausentes y enfermos, y lo están verificando Sucesivamente.”